vhost: force vhost off for non-MSI guests

When MSI is off, each interrupt needs to be bounced through the io thread when it's set/cleared, so vhost-net causes more context switches and higher CPU utilization than userspace virtio which handles networking in the same thread.

We'll need to fix this by adding level irq support in kvm irqfd, for now disable vhost-net in these configurations.

Added a vhostforce flag to force vhost-net back on.

Signed-off-by Michael S. Tsirkin<[email protected]>

---

Untested, I'll send a pull request later after
some testing assuming we've resolved the
command line comments to everyone's satisfaction.

Changes since v1:
added vhostforce to enable vhost for non-MSI guests

  hw/vhost.c      |   16 +++++++++++-----
  hw/vhost.h      |    3 ++-
  hw/vhost_net.c  |    8 +++++---
  hw/vhost_net.h  |    2 +-
  hw/virtio-net.c |    6 ++++--
  hw/virtio-pci.c |    6 +++++-
  hw/virtio.h     |    4 +++-
  net/tap.c       |    6 ++++--
  qemu-options.hx |    4 +++-
  9 files changed, 38 insertions(+), 17 deletions(-)

hw/vhost.c +11 -5

index 6082da2..72df75f 100644
@@ -581,7 +581,7 @@ static void vhost_virtqueue_cleanup(struct vhost_dev *dev,
                                0, virtio_queue_get_desc_size(vdev, idx));
  }
 
-int vhost_dev_init(struct vhost_dev *hdev, int devfd)
+int vhost_dev_init(struct vhost_dev *hdev, int devfd, bool force)
  {
      uint64_t features;
      int r;

@@ -613,6 +613,7 @@ int vhost_dev_init(struct vhost_dev *hdev, int devfd)
      hdev->log_enabled = false;
      hdev->started = false;
      cpu_register_phys_memory_client(&hdev->client);
+    hdev->force = force;
      return 0;
  fail:
      r = -errno;

@@ -636,9 +637,12 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
          goto fail;
      }
 
-    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, true);
+    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, true,
+                                           hdev->force);
      if (r<  0) {
-        fprintf(stderr, "Error binding guest notifier: %d\n", -r);
+	if (r != -EVIRTIO_DISABLED) {
+		fprintf(stderr, "Error binding guest notifier: %d\n", -r);
+	}
          goto fail_notifiers;
      }
 

@@ -686,7 +690,8 @@ fail_vq:
      }
  fail_mem:
  fail_features:
-    vdev->binding->set_guest_notifiers(vdev->binding_opaque, false);
+    vdev->binding->set_guest_notifiers(vdev->binding_opaque, false,
+                                       hdev->force);
  fail_notifiers:
  fail:
      return r;

@@ -704,7 +709,8 @@ void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev)
      }
      vhost_client_sync_dirty_bitmap(&hdev->client, 0,
                                     (target_phys_addr_t)~0x0ull);
-    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, false);
+    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, false,
+                                           hdev->force);
      if (r<  0) {
          fprintf(stderr, "vhost guest notifier cleanup failed: %d\n", r);
          fflush(stderr);

hw/vhost.h +2 -1

index 86dd834..d5d4d86 100644
@@ -38,9 +38,10 @@ struct vhost_dev {
      bool log_enabled;
      vhost_log_chunk_t *log;
      unsigned long long log_size;
+    bool force;
  };
 
-int vhost_dev_init(struct vhost_dev *hdev, int devfd);
+int vhost_dev_init(struct vhost_dev *hdev, int devfd, bool force);
  void vhost_dev_cleanup(struct vhost_dev *hdev);
  int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev);
  void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev);

hw/vhost_net.c +5 -3

index c068be1..4f57271 100644
@@ -81,7 +81,8 @@ static int vhost_net_get_fd(VLANClientState *backend)
      }
  }
 
-struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
+struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd,
+                                 bool force)
  {
      int r;
      struct vhost_net *net = qemu_malloc(sizeof *net);

@@ -98,7 +99,7 @@ struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
          (1<<  VHOST_NET_F_VIRTIO_NET_HDR);
      net->backend = r;
 
-    r = vhost_dev_init(&net->dev, devfd);
+    r = vhost_dev_init(&net->dev, devfd, force);
      if (r<  0) {
          goto fail;
      }

@@ -188,7 +189,8 @@ void vhost_net_cleanup(struct vhost_net *net)
      qemu_free(net);
  }
  #else
-struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
+struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd,
+                                 bool force)
  {
  	return NULL;
  }

hw/vhost_net.h +1 -1

index 6c18ff7..8435094 100644
@@ -6,7 +6,7 @@
  struct vhost_net;
  typedef struct vhost_net VHostNetState;
 
-VHostNetState *vhost_net_init(VLANClientState *backend, int devfd);
+VHostNetState *vhost_net_init(VLANClientState *backend, int devfd, bool force);
 
  int vhost_net_start(VHostNetState *net, VirtIODevice *dev);
  void vhost_net_stop(VHostNetState *net, VirtIODevice *dev);

hw/virtio-net.c +4 -2

index ccb3e63..5de3fee 100644
@@ -121,8 +121,10 @@ static void virtio_net_vhost_status(VirtIONet *n, uint8_t status)
      if (!n->vhost_started) {
          int r = vhost_net_start(tap_get_vhost_net(n->nic->nc.peer),&n->vdev);
          if (r<  0) {
-            error_report("unable to start vhost net: %d: "
-                         "falling back on userspace virtio", -r);
+            if (r != -EVIRTIO_DISABLED) {
+                error_report("unable to start vhost net: %d: "
+                             "falling back on userspace virtio", -r);
+            }
          } else {
              n->vhost_started = 1;
          }

hw/virtio-pci.c +5 -1

index d07ff97..c93ef11 100644
@@ -595,12 +595,16 @@ static int virtio_pci_set_guest_notifier(void *opaque, int n, bool assign)
      return 0;
  }
 
-static int virtio_pci_set_guest_notifiers(void *opaque, bool assign)
+static int virtio_pci_set_guest_notifiers(void *opaque, bool assign, bool force)
  {
      VirtIOPCIProxy *proxy = opaque;
      VirtIODevice *vdev = proxy->vdev;
      int r, n;
 
+    if (assign&&  !force&&  !msix_enabled(&proxy->pci_dev)) {
+	    return -EVIRTIO_DISABLED;
+    }
+
      for (n = 0; n<  VIRTIO_PCI_QUEUE_MAX; n++) {
          if (!virtio_queue_get_num(vdev, n)) {
              break;

hw/virtio.h +3 -1

index d8546d5..156c9a3 100644
@@ -93,11 +93,13 @@ typedef struct {
      int (*load_config)(void * opaque, QEMUFile *f);
      int (*load_queue)(void * opaque, int n, QEMUFile *f);
      unsigned (*get_features)(void * opaque);
-    int (*set_guest_notifiers)(void * opaque, bool assigned);
+    int (*set_guest_notifiers)(void * opaque, bool assigned, bool force);
      int (*set_host_notifier)(void * opaque, int n, bool assigned);
      void (*vmstate_change)(void * opaque, bool running);
  } VirtIOBindings;
 
+#define EVIRTIO_DISABLED ERANGE
+
  #define VIRTIO_PCI_QUEUE_MAX 64
 
  #define VIRTIO_NO_VECTOR 0xffff

net/tap.c +4 -2

index eada34a..b8cd252 100644
@@ -491,8 +491,10 @@ int net_init_tap(QemuOpts *opts, Monitor *mon, const char *name, VLANState *vlan
          }
      }
 
-    if (qemu_opt_get_bool(opts, "vhost", !!qemu_opt_get(opts, "vhostfd"))) {
+    if (qemu_opt_get_bool(opts, "vhost", !!qemu_opt_get(opts, "vhostfd") ||
+                          qemu_opt_get_bool(opts, "vhostforce", false))) {
          int vhostfd, r;
+        bool force = qemu_opt_get_bool(opts, "vhostforce", false);
          if (qemu_opt_get(opts, "vhostfd")) {
              r = net_handle_fd_param(mon, qemu_opt_get(opts, "vhostfd"));
              if (r == -1) {

@@ -502,7 +504,7 @@ int net_init_tap(QemuOpts *opts, Monitor *mon, const char *name, VLANState *vlan
          } else {
              vhostfd = -1;
          }
-        s->vhost_net = vhost_net_init(&s->nc, vhostfd);
+        s->vhost_net = vhost_net_init(&s->nc, vhostfd, force);
          if (!s->vhost_net) {
              error_report("vhost-net requested but could not be initialized");
              return -1;

qemu-options.hx +3 -1

index 898561d..11c93a2 100644
@@ -1050,7 +1050,7 @@ DEF("net", HAS_ARG, QEMU_OPTION_net,
      "-net tap[,vlan=n][,name=str],ifname=name\n"
      "                connect the host TAP network interface to VLAN 'n'\n"
  #else
-    "-net tap[,vlan=n][,name=str][,fd=h][,ifname=name][,script=file][,downscript=dfile][,sndbuf=nbytes][,vnet_hdr=on|off][,vhost=on|off][,vhostfd=h]\n"
+    "-net tap[,vlan=n][,name=str][,fd=h][,ifname=name][,script=file][,downscript=dfile][,sndbuf=nbytes][,vnet_hdr=on|off][,vhost=on|off][,vhostfd=h][,vhostforce=on|off]\n"
      "                connect the host TAP network interface to VLAN 'n' and use the\n"
      "                network scripts 'file' (default=" DEFAULT_NETWORK_SCRIPT ")\n"
      "                and 'dfile' (default=" DEFAULT_NETWORK_DOWN_SCRIPT ")\n"

@@ -1061,6 +1061,8 @@ DEF("net", HAS_ARG, QEMU_OPTION_net,
      "                use vnet_hdr=off to avoid enabling the IFF_VNET_HDR tap flag\n"
      "                use vnet_hdr=on to make the lack of IFF_VNET_HDR support an error condition\n"
      "                use vhost=on to enable experimental in kernel accelerator\n"
+    "                    (only has effect for virtio guests which use MSIX)\n"
+    "                use vhostforce=on to force vhost on for non-MSIX virtio guests\n"
      "                use 'vhostfd=h' to connect to an already opened vhost net device\n"
  #endif
      "-net socket[,vlan=n][,name=str][,fd=h][,listen=[host]:port][,connect=host:port]\n"

When MSI is off, each interrupt needs to be bounced through the io thread when it's set/cleared, so vhost-net causes more context switches and higher CPU utilization than userspace virtio which handles networking in the same thread.

We'll need to fix this by adding level irq support in kvm irqfd, for now disable vhost-net in these configurations.

Added a vhostforce flag to force vhost-net back on.

Signed-off-by Michael S. Tsirkin <[email protected]>

---

Untested, I'll send a pull request later after
some testing assuming we've resolved the
command line comments to everyone's satisfaction.

Changes since v1:
added vhostforce to enable vhost for non-MSI guests

 hw/vhost.c      |   16 +++++++++++-----
 hw/vhost.h      |    3 ++-
 hw/vhost_net.c  |    8 +++++---
 hw/vhost_net.h  |    2 +-
 hw/virtio-net.c |    6 ++++--
 hw/virtio-pci.c |    6 +++++-
 hw/virtio.h     |    4 +++-
 net/tap.c       |    6 ++++--
 qemu-options.hx |    4 +++-
 9 files changed, 38 insertions(+), 17 deletions(-)

hw/vhost.c +11 -5

index 6082da2..72df75f 100644
@@ -581,7 +581,7 @@ static void vhost_virtqueue_cleanup(struct vhost_dev *dev,
                               0, virtio_queue_get_desc_size(vdev, idx));
 }
 
-int vhost_dev_init(struct vhost_dev *hdev, int devfd)
+int vhost_dev_init(struct vhost_dev *hdev, int devfd, bool force)
 {
     uint64_t features;
     int r;

@@ -613,6 +613,7 @@ int vhost_dev_init(struct vhost_dev *hdev, int devfd)
     hdev->log_enabled = false;
     hdev->started = false;
     cpu_register_phys_memory_client(&hdev->client);
+    hdev->force = force;
     return 0;
 fail:
     r = -errno;

@@ -636,9 +637,12 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
         goto fail;
     }
 
-    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, true);
+    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, true,
+                                           hdev->force);
     if (r < 0) {
-        fprintf(stderr, "Error binding guest notifier: %d\n", -r);
+	if (r != -EVIRTIO_DISABLED) {
+		fprintf(stderr, "Error binding guest notifier: %d\n", -r);
+	}
         goto fail_notifiers;
     }
 

@@ -686,7 +690,8 @@ fail_vq:
     }
 fail_mem:
 fail_features:
-    vdev->binding->set_guest_notifiers(vdev->binding_opaque, false);
+    vdev->binding->set_guest_notifiers(vdev->binding_opaque, false,
+                                       hdev->force);
 fail_notifiers:
 fail:
     return r;

@@ -704,7 +709,8 @@ void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev)
     }
     vhost_client_sync_dirty_bitmap(&hdev->client, 0,
                                    (target_phys_addr_t)~0x0ull);
-    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, false);
+    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, false,
+                                           hdev->force);
     if (r < 0) {
         fprintf(stderr, "vhost guest notifier cleanup failed: %d\n", r);
         fflush(stderr);

hw/vhost.h +2 -1

index 86dd834..d5d4d86 100644
@@ -38,9 +38,10 @@ struct vhost_dev {
     bool log_enabled;
     vhost_log_chunk_t *log;
     unsigned long long log_size;
+    bool force;
 };
 
-int vhost_dev_init(struct vhost_dev *hdev, int devfd);
+int vhost_dev_init(struct vhost_dev *hdev, int devfd, bool force);
 void vhost_dev_cleanup(struct vhost_dev *hdev);
 int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev);
 void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev);

hw/vhost_net.c +5 -3

index c068be1..4f57271 100644
@@ -81,7 +81,8 @@ static int vhost_net_get_fd(VLANClientState *backend)
     }
 }
 
-struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
+struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd,
+                                 bool force)
 {
     int r;
     struct vhost_net *net = qemu_malloc(sizeof *net);

@@ -98,7 +99,7 @@ struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
         (1 << VHOST_NET_F_VIRTIO_NET_HDR);
     net->backend = r;
 
-    r = vhost_dev_init(&net->dev, devfd);
+    r = vhost_dev_init(&net->dev, devfd, force);
     if (r < 0) {
         goto fail;
     }

@@ -188,7 +189,8 @@ void vhost_net_cleanup(struct vhost_net *net)
     qemu_free(net);
 }
 #else
-struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
+struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd,
+                                 bool force)
 {
 	return NULL;
 }

hw/vhost_net.h +1 -1

index 6c18ff7..8435094 100644
@@ -6,7 +6,7 @@
 struct vhost_net;
 typedef struct vhost_net VHostNetState;
 
-VHostNetState *vhost_net_init(VLANClientState *backend, int devfd);
+VHostNetState *vhost_net_init(VLANClientState *backend, int devfd, bool force);
 
 int vhost_net_start(VHostNetState *net, VirtIODevice *dev);
 void vhost_net_stop(VHostNetState *net, VirtIODevice *dev);

hw/virtio-net.c +4 -2

index ccb3e63..5de3fee 100644
@@ -121,8 +121,10 @@ static void virtio_net_vhost_status(VirtIONet *n, uint8_t status)
     if (!n->vhost_started) {
         int r = vhost_net_start(tap_get_vhost_net(n->nic->nc.peer), &n->vdev);
         if (r < 0) {
-            error_report("unable to start vhost net: %d: "
-                         "falling back on userspace virtio", -r);
+            if (r != -EVIRTIO_DISABLED) {
+                error_report("unable to start vhost net: %d: "
+                             "falling back on userspace virtio", -r);
+            }
         } else {
             n->vhost_started = 1;
         }

hw/virtio-pci.c +5 -1

index d07ff97..c93ef11 100644
@@ -595,12 +595,16 @@ static int virtio_pci_set_guest_notifier(void *opaque, int n, bool assign)
     return 0;
 }
 
-static int virtio_pci_set_guest_notifiers(void *opaque, bool assign)
+static int virtio_pci_set_guest_notifiers(void *opaque, bool assign, bool force)
 {
     VirtIOPCIProxy *proxy = opaque;
     VirtIODevice *vdev = proxy->vdev;
     int r, n;
 
+    if (assign && !force && !msix_enabled(&proxy->pci_dev)) {
+	    return -EVIRTIO_DISABLED;
+    }
+
     for (n = 0; n < VIRTIO_PCI_QUEUE_MAX; n++) {
         if (!virtio_queue_get_num(vdev, n)) {
             break;

hw/virtio.h +3 -1

index d8546d5..156c9a3 100644
@@ -93,11 +93,13 @@ typedef struct {
     int (*load_config)(void * opaque, QEMUFile *f);
     int (*load_queue)(void * opaque, int n, QEMUFile *f);
     unsigned (*get_features)(void * opaque);
-    int (*set_guest_notifiers)(void * opaque, bool assigned);
+    int (*set_guest_notifiers)(void * opaque, bool assigned, bool force);
     int (*set_host_notifier)(void * opaque, int n, bool assigned);
     void (*vmstate_change)(void * opaque, bool running);
 } VirtIOBindings;
 
+#define EVIRTIO_DISABLED ERANGE
+
 #define VIRTIO_PCI_QUEUE_MAX 64
 
 #define VIRTIO_NO_VECTOR 0xffff

net/tap.c +4 -2

index eada34a..b8cd252 100644
@@ -491,8 +491,10 @@ int net_init_tap(QemuOpts *opts, Monitor *mon, const char *name, VLANState *vlan
         }
     }
 
-    if (qemu_opt_get_bool(opts, "vhost", !!qemu_opt_get(opts, "vhostfd"))) {
+    if (qemu_opt_get_bool(opts, "vhost", !!qemu_opt_get(opts, "vhostfd") ||
+                          qemu_opt_get_bool(opts, "vhostforce", false))) {
         int vhostfd, r;
+        bool force = qemu_opt_get_bool(opts, "vhostforce", false);
         if (qemu_opt_get(opts, "vhostfd")) {
             r = net_handle_fd_param(mon, qemu_opt_get(opts, "vhostfd"));
             if (r == -1) {

@@ -502,7 +504,7 @@ int net_init_tap(QemuOpts *opts, Monitor *mon, const char *name, VLANState *vlan
         } else {
             vhostfd = -1;
         }
-        s->vhost_net = vhost_net_init(&s->nc, vhostfd);
+        s->vhost_net = vhost_net_init(&s->nc, vhostfd, force);
         if (!s->vhost_net) {
             error_report("vhost-net requested but could not be initialized");
             return -1;

qemu-options.hx +3 -1

index 898561d..11c93a2 100644
@@ -1050,7 +1050,7 @@ DEF("net", HAS_ARG, QEMU_OPTION_net,
     "-net tap[,vlan=n][,name=str],ifname=name\n"
     "                connect the host TAP network interface to VLAN 'n'\n"
 #else
-    "-net tap[,vlan=n][,name=str][,fd=h][,ifname=name][,script=file][,downscript=dfile][,sndbuf=nbytes][,vnet_hdr=on|off][,vhost=on|off][,vhostfd=h]\n"
+    "-net tap[,vlan=n][,name=str][,fd=h][,ifname=name][,script=file][,downscript=dfile][,sndbuf=nbytes][,vnet_hdr=on|off][,vhost=on|off][,vhostfd=h][,vhostforce=on|off]\n"
     "                connect the host TAP network interface to VLAN 'n' and use the\n"
     "                network scripts 'file' (default=" DEFAULT_NETWORK_SCRIPT ")\n"
     "                and 'dfile' (default=" DEFAULT_NETWORK_DOWN_SCRIPT ")\n"

@@ -1061,6 +1061,8 @@ DEF("net", HAS_ARG, QEMU_OPTION_net,
     "                use vnet_hdr=off to avoid enabling the IFF_VNET_HDR tap flag\n"
     "                use vnet_hdr=on to make the lack of IFF_VNET_HDR support an error condition\n"
     "                use vhost=on to enable experimental in kernel accelerator\n"
+    "                    (only has effect for virtio guests which use MSIX)\n"
+    "                use vhostforce=on to force vhost on for non-MSIX virtio guests\n"
     "                use 'vhostfd=h' to connect to an already opened vhost net device\n"
 #endif
     "-net socket[,vlan=n][,name=str][,fd=h][,listen=[host]:port][,connect=host:port]\n"

Quoted 11 lines · Michael S. Tsirkin

When MSI is off, each interrupt needs to be bounced through the io thread when it's set/cleared, so vhost-net causes more context switches and higher CPU utilization than userspace virtio which handles networking in the same thread.

We'll need to fix this by adding level irq support in kvm irqfd, for now disable vhost-net in these configurations.

Added a vhostforce flag to force vhost-net back on.

Signed-off-by Michael S. Tsirkin <[email protected]>

Quoted 8 lines

---

Untested, I'll send a pull request later after
some testing assuming we've resolved the
command line comments to everyone's satisfaction.

Changes since v1:
added vhostforce to enable vhost for non-MSI guests

I'm still not thrilled with the errno hack. If this is really a generic problem, we should better architect the set_guest_notifiers() callback, the force option feels like a band-aide.
I'm still curious if we can't
be more dynamic about how we setup the set_guest_notifiers() function pointer so it only gets set if (force || msix_enabled()) and we can limit some of the impact here to vhost. Thanks,

Alex

Quoted 234 lines

 hw/vhost.c      |   16 +++++++++++-----
 hw/vhost.h      |    3 ++-
 hw/vhost_net.c  |    8 +++++---
 hw/vhost_net.h  |    2 +-
 hw/virtio-net.c |    6 ++++--
 hw/virtio-pci.c |    6 +++++-
 hw/virtio.h     |    4 +++-
 net/tap.c       |    6 ++++--
 qemu-options.hx |    4 +++-
 9 files changed, 38 insertions(+), 17 deletions(-)

hw/vhost.c +11 -5

index 6082da2..72df75f 100644
@@ -581,7 +581,7 @@ static void vhost_virtqueue_cleanup(struct vhost_dev *dev,
                               0, virtio_queue_get_desc_size(vdev, idx));
 }
 
-int vhost_dev_init(struct vhost_dev *hdev, int devfd)
+int vhost_dev_init(struct vhost_dev *hdev, int devfd, bool force)
 {
     uint64_t features;
     int r;

@@ -613,6 +613,7 @@ int vhost_dev_init(struct vhost_dev *hdev, int devfd)
     hdev->log_enabled = false;
     hdev->started = false;
     cpu_register_phys_memory_client(&hdev->client);
+    hdev->force = force;
     return 0;
 fail:
     r = -errno;

@@ -636,9 +637,12 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
         goto fail;
     }
 
-    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, true);
+    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, true,
+                                           hdev->force);
     if (r < 0) {
-        fprintf(stderr, "Error binding guest notifier: %d\n", -r);
+	if (r != -EVIRTIO_DISABLED) {
+		fprintf(stderr, "Error binding guest notifier: %d\n", -r);
+	}
         goto fail_notifiers;
     }
 

@@ -686,7 +690,8 @@ fail_vq:
     }
 fail_mem:
 fail_features:
-    vdev->binding->set_guest_notifiers(vdev->binding_opaque, false);
+    vdev->binding->set_guest_notifiers(vdev->binding_opaque, false,
+                                       hdev->force);
 fail_notifiers:
 fail:
     return r;

@@ -704,7 +709,8 @@ void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev)
     }
     vhost_client_sync_dirty_bitmap(&hdev->client, 0,
                                    (target_phys_addr_t)~0x0ull);
-    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, false);
+    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, false,
+                                           hdev->force);
     if (r < 0) {
         fprintf(stderr, "vhost guest notifier cleanup failed: %d\n", r);
         fflush(stderr);

hw/vhost.h +2 -1

index 86dd834..d5d4d86 100644
@@ -38,9 +38,10 @@ struct vhost_dev {
     bool log_enabled;
     vhost_log_chunk_t *log;
     unsigned long long log_size;
+    bool force;
 };
 
-int vhost_dev_init(struct vhost_dev *hdev, int devfd);
+int vhost_dev_init(struct vhost_dev *hdev, int devfd, bool force);
 void vhost_dev_cleanup(struct vhost_dev *hdev);
 int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev);
 void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev);

hw/vhost_net.c +5 -3

index c068be1..4f57271 100644
@@ -81,7 +81,8 @@ static int vhost_net_get_fd(VLANClientState *backend)
     }
 }
 
-struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
+struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd,
+                                 bool force)
 {
     int r;
     struct vhost_net *net = qemu_malloc(sizeof *net);

@@ -98,7 +99,7 @@ struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
         (1 << VHOST_NET_F_VIRTIO_NET_HDR);
     net->backend = r;
 
-    r = vhost_dev_init(&net->dev, devfd);
+    r = vhost_dev_init(&net->dev, devfd, force);
     if (r < 0) {
         goto fail;
     }

@@ -188,7 +189,8 @@ void vhost_net_cleanup(struct vhost_net *net)
     qemu_free(net);
 }
 #else
-struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
+struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd,
+                                 bool force)
 {
 	return NULL;
 }

hw/vhost_net.h +1 -1

index 6c18ff7..8435094 100644
@@ -6,7 +6,7 @@
 struct vhost_net;
 typedef struct vhost_net VHostNetState;
 
-VHostNetState *vhost_net_init(VLANClientState *backend, int devfd);
+VHostNetState *vhost_net_init(VLANClientState *backend, int devfd, bool force);
 
 int vhost_net_start(VHostNetState *net, VirtIODevice *dev);
 void vhost_net_stop(VHostNetState *net, VirtIODevice *dev);

hw/virtio-net.c +4 -2

index ccb3e63..5de3fee 100644
@@ -121,8 +121,10 @@ static void virtio_net_vhost_status(VirtIONet *n, uint8_t status)
     if (!n->vhost_started) {
         int r = vhost_net_start(tap_get_vhost_net(n->nic->nc.peer), &n->vdev);
         if (r < 0) {
-            error_report("unable to start vhost net: %d: "
-                         "falling back on userspace virtio", -r);
+            if (r != -EVIRTIO_DISABLED) {
+                error_report("unable to start vhost net: %d: "
+                             "falling back on userspace virtio", -r);
+            }
         } else {
             n->vhost_started = 1;
         }

hw/virtio-pci.c +5 -1

index d07ff97..c93ef11 100644
@@ -595,12 +595,16 @@ static int virtio_pci_set_guest_notifier(void *opaque, int n, bool assign)
     return 0;
 }
 
-static int virtio_pci_set_guest_notifiers(void *opaque, bool assign)
+static int virtio_pci_set_guest_notifiers(void *opaque, bool assign, bool force)
 {
     VirtIOPCIProxy *proxy = opaque;
     VirtIODevice *vdev = proxy->vdev;
     int r, n;
 
+    if (assign && !force && !msix_enabled(&proxy->pci_dev)) {
+	    return -EVIRTIO_DISABLED;
+    }
+
     for (n = 0; n < VIRTIO_PCI_QUEUE_MAX; n++) {
         if (!virtio_queue_get_num(vdev, n)) {
             break;

hw/virtio.h +3 -1

index d8546d5..156c9a3 100644
@@ -93,11 +93,13 @@ typedef struct {
     int (*load_config)(void * opaque, QEMUFile *f);
     int (*load_queue)(void * opaque, int n, QEMUFile *f);
     unsigned (*get_features)(void * opaque);
-    int (*set_guest_notifiers)(void * opaque, bool assigned);
+    int (*set_guest_notifiers)(void * opaque, bool assigned, bool force);
     int (*set_host_notifier)(void * opaque, int n, bool assigned);
     void (*vmstate_change)(void * opaque, bool running);
 } VirtIOBindings;
 
+#define EVIRTIO_DISABLED ERANGE
+
 #define VIRTIO_PCI_QUEUE_MAX 64
 
 #define VIRTIO_NO_VECTOR 0xffff

net/tap.c +4 -2

index eada34a..b8cd252 100644
@@ -491,8 +491,10 @@ int net_init_tap(QemuOpts *opts, Monitor *mon, const char *name, VLANState *vlan
         }
     }
 
-    if (qemu_opt_get_bool(opts, "vhost", !!qemu_opt_get(opts, "vhostfd"))) {
+    if (qemu_opt_get_bool(opts, "vhost", !!qemu_opt_get(opts, "vhostfd") ||
+                          qemu_opt_get_bool(opts, "vhostforce", false))) {
         int vhostfd, r;
+        bool force = qemu_opt_get_bool(opts, "vhostforce", false);
         if (qemu_opt_get(opts, "vhostfd")) {
             r = net_handle_fd_param(mon, qemu_opt_get(opts, "vhostfd"));
             if (r == -1) {

@@ -502,7 +504,7 @@ int net_init_tap(QemuOpts *opts, Monitor *mon, const char *name, VLANState *vlan
         } else {
             vhostfd = -1;
         }
-        s->vhost_net = vhost_net_init(&s->nc, vhostfd);
+        s->vhost_net = vhost_net_init(&s->nc, vhostfd, force);
         if (!s->vhost_net) {
             error_report("vhost-net requested but could not be initialized");
             return -1;

qemu-options.hx +3 -1

index 898561d..11c93a2 100644
@@ -1050,7 +1050,7 @@ DEF("net", HAS_ARG, QEMU_OPTION_net,
     "-net tap[,vlan=n][,name=str],ifname=name\n"
     "                connect the host TAP network interface to VLAN 'n'\n"
 #else
-    "-net tap[,vlan=n][,name=str][,fd=h][,ifname=name][,script=file][,downscript=dfile][,sndbuf=nbytes][,vnet_hdr=on|off][,vhost=on|off][,vhostfd=h]\n"
+    "-net tap[,vlan=n][,name=str][,fd=h][,ifname=name][,script=file][,downscript=dfile][,sndbuf=nbytes][,vnet_hdr=on|off][,vhost=on|off][,vhostfd=h][,vhostforce=on|off]\n"
     "                connect the host TAP network interface to VLAN 'n' and use the\n"
     "                network scripts 'file' (default=" DEFAULT_NETWORK_SCRIPT ")\n"
     "                and 'dfile' (default=" DEFAULT_NETWORK_DOWN_SCRIPT ")\n"

@@ -1061,6 +1061,8 @@ DEF("net", HAS_ARG, QEMU_OPTION_net,
     "                use vnet_hdr=off to avoid enabling the IFF_VNET_HDR tap flag\n"
     "                use vnet_hdr=on to make the lack of IFF_VNET_HDR support an error condition\n"
     "                use vhost=on to enable experimental in kernel accelerator\n"
+    "                    (only has effect for virtio guests which use MSIX)\n"
+    "                use vhostforce=on to force vhost on for non-MSIX virtio guests\n"
     "                use 'vhostfd=h' to connect to an already opened vhost net device\n"
 #endif
     "-net socket[,vlan=n][,name=str][,fd=h][,listen=[host]:port][,connect=host:port]\n"

Quoted 29 lines · Alex Williamson

Quoted 11 lines · Michael S. Tsirkin

When MSI is off, each interrupt needs to be bounced through the io thread when it's set/cleared, so vhost-net causes more context switches and higher CPU utilization than userspace virtio which handles networking in the same thread.

We'll need to fix this by adding level irq support in kvm irqfd, for now disable vhost-net in these configurations.

Added a vhostforce flag to force vhost-net back on.

Signed-off-by Michael S. Tsirkin <[email protected]>

Quoted 8 lines

---

Untested, I'll send a pull request later after
some testing assuming we've resolved the
command line comments to everyone's satisfaction.

Changes since v1:
added vhostforce to enable vhost for non-MSI guests

I'm still not thrilled with the errno hack. If this is really a generic problem, we should better architect the set_guest_notifiers() callback, the force option feels like a band-aide.
I'm still curious if we can't
be more dynamic about how we setup the set_guest_notifiers() function pointer so it only gets set if (force || msix_enabled()) and we can limit some of the impact here to vhost. Thanks,

Quoted 1 line

Alex

I'm not entirely happy with the API either, but I'm sure playing with the function pointer is not the solution we are looking for:

msix is enabled/disabled dynamically. force is static but it is something net backend knows about, virtio pci doesn't. So I suspect modifying the callback on the fly will become messy quite quickly.

Well, this API will be easy to tweak after the fact, there's a single user after all.

Quoted 236 lines

Quoted 234 lines

 hw/vhost.c      |   16 +++++++++++-----
 hw/vhost.h      |    3 ++-
 hw/vhost_net.c  |    8 +++++---
 hw/vhost_net.h  |    2 +-
 hw/virtio-net.c |    6 ++++--
 hw/virtio-pci.c |    6 +++++-
 hw/virtio.h     |    4 +++-
 net/tap.c       |    6 ++++--
 qemu-options.hx |    4 +++-
 9 files changed, 38 insertions(+), 17 deletions(-)

hw/vhost.c +11 -5

index 6082da2..72df75f 100644
@@ -581,7 +581,7 @@ static void vhost_virtqueue_cleanup(struct vhost_dev *dev,
                               0, virtio_queue_get_desc_size(vdev, idx));
 }
 
-int vhost_dev_init(struct vhost_dev *hdev, int devfd)
+int vhost_dev_init(struct vhost_dev *hdev, int devfd, bool force)
 {
     uint64_t features;
     int r;

@@ -613,6 +613,7 @@ int vhost_dev_init(struct vhost_dev *hdev, int devfd)
     hdev->log_enabled = false;
     hdev->started = false;
     cpu_register_phys_memory_client(&hdev->client);
+    hdev->force = force;
     return 0;
 fail:
     r = -errno;

@@ -636,9 +637,12 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
         goto fail;
     }
 
-    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, true);
+    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, true,
+                                           hdev->force);
     if (r < 0) {
-        fprintf(stderr, "Error binding guest notifier: %d\n", -r);
+	if (r != -EVIRTIO_DISABLED) {
+		fprintf(stderr, "Error binding guest notifier: %d\n", -r);
+	}
         goto fail_notifiers;
     }
 

@@ -686,7 +690,8 @@ fail_vq:
     }
 fail_mem:
 fail_features:
-    vdev->binding->set_guest_notifiers(vdev->binding_opaque, false);
+    vdev->binding->set_guest_notifiers(vdev->binding_opaque, false,
+                                       hdev->force);
 fail_notifiers:
 fail:
     return r;

@@ -704,7 +709,8 @@ void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev)
     }
     vhost_client_sync_dirty_bitmap(&hdev->client, 0,
                                    (target_phys_addr_t)~0x0ull);
-    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, false);
+    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, false,
+                                           hdev->force);
     if (r < 0) {
         fprintf(stderr, "vhost guest notifier cleanup failed: %d\n", r);
         fflush(stderr);

hw/vhost.h +2 -1

index 86dd834..d5d4d86 100644
@@ -38,9 +38,10 @@ struct vhost_dev {
     bool log_enabled;
     vhost_log_chunk_t *log;
     unsigned long long log_size;
+    bool force;
 };
 
-int vhost_dev_init(struct vhost_dev *hdev, int devfd);
+int vhost_dev_init(struct vhost_dev *hdev, int devfd, bool force);
 void vhost_dev_cleanup(struct vhost_dev *hdev);
 int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev);
 void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev);

hw/vhost_net.c +5 -3

index c068be1..4f57271 100644
@@ -81,7 +81,8 @@ static int vhost_net_get_fd(VLANClientState *backend)
     }
 }
 
-struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
+struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd,
+                                 bool force)
 {
     int r;
     struct vhost_net *net = qemu_malloc(sizeof *net);

@@ -98,7 +99,7 @@ struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
         (1 << VHOST_NET_F_VIRTIO_NET_HDR);
     net->backend = r;
 
-    r = vhost_dev_init(&net->dev, devfd);
+    r = vhost_dev_init(&net->dev, devfd, force);
     if (r < 0) {
         goto fail;
     }

@@ -188,7 +189,8 @@ void vhost_net_cleanup(struct vhost_net *net)
     qemu_free(net);
 }
 #else
-struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
+struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd,
+                                 bool force)
 {
 	return NULL;
 }

hw/vhost_net.h +1 -1

index 6c18ff7..8435094 100644
@@ -6,7 +6,7 @@
 struct vhost_net;
 typedef struct vhost_net VHostNetState;
 
-VHostNetState *vhost_net_init(VLANClientState *backend, int devfd);
+VHostNetState *vhost_net_init(VLANClientState *backend, int devfd, bool force);
 
 int vhost_net_start(VHostNetState *net, VirtIODevice *dev);
 void vhost_net_stop(VHostNetState *net, VirtIODevice *dev);

hw/virtio-net.c +4 -2

index ccb3e63..5de3fee 100644
@@ -121,8 +121,10 @@ static void virtio_net_vhost_status(VirtIONet *n, uint8_t status)
     if (!n->vhost_started) {
         int r = vhost_net_start(tap_get_vhost_net(n->nic->nc.peer), &n->vdev);
         if (r < 0) {
-            error_report("unable to start vhost net: %d: "
-                         "falling back on userspace virtio", -r);
+            if (r != -EVIRTIO_DISABLED) {
+                error_report("unable to start vhost net: %d: "
+                             "falling back on userspace virtio", -r);
+            }
         } else {
             n->vhost_started = 1;
         }

hw/virtio-pci.c +5 -1

index d07ff97..c93ef11 100644
@@ -595,12 +595,16 @@ static int virtio_pci_set_guest_notifier(void *opaque, int n, bool assign)
     return 0;
 }
 
-static int virtio_pci_set_guest_notifiers(void *opaque, bool assign)
+static int virtio_pci_set_guest_notifiers(void *opaque, bool assign, bool force)
 {
     VirtIOPCIProxy *proxy = opaque;
     VirtIODevice *vdev = proxy->vdev;
     int r, n;
 
+    if (assign && !force && !msix_enabled(&proxy->pci_dev)) {
+	    return -EVIRTIO_DISABLED;
+    }
+
     for (n = 0; n < VIRTIO_PCI_QUEUE_MAX; n++) {
         if (!virtio_queue_get_num(vdev, n)) {
             break;

hw/virtio.h +3 -1

index d8546d5..156c9a3 100644
@@ -93,11 +93,13 @@ typedef struct {
     int (*load_config)(void * opaque, QEMUFile *f);
     int (*load_queue)(void * opaque, int n, QEMUFile *f);
     unsigned (*get_features)(void * opaque);
-    int (*set_guest_notifiers)(void * opaque, bool assigned);
+    int (*set_guest_notifiers)(void * opaque, bool assigned, bool force);
     int (*set_host_notifier)(void * opaque, int n, bool assigned);
     void (*vmstate_change)(void * opaque, bool running);
 } VirtIOBindings;
 
+#define EVIRTIO_DISABLED ERANGE
+
 #define VIRTIO_PCI_QUEUE_MAX 64
 
 #define VIRTIO_NO_VECTOR 0xffff

net/tap.c +4 -2

index eada34a..b8cd252 100644
@@ -491,8 +491,10 @@ int net_init_tap(QemuOpts *opts, Monitor *mon, const char *name, VLANState *vlan
         }
     }
 
-    if (qemu_opt_get_bool(opts, "vhost", !!qemu_opt_get(opts, "vhostfd"))) {
+    if (qemu_opt_get_bool(opts, "vhost", !!qemu_opt_get(opts, "vhostfd") ||
+                          qemu_opt_get_bool(opts, "vhostforce", false))) {
         int vhostfd, r;
+        bool force = qemu_opt_get_bool(opts, "vhostforce", false);
         if (qemu_opt_get(opts, "vhostfd")) {
             r = net_handle_fd_param(mon, qemu_opt_get(opts, "vhostfd"));
             if (r == -1) {

@@ -502,7 +504,7 @@ int net_init_tap(QemuOpts *opts, Monitor *mon, const char *name, VLANState *vlan
         } else {
             vhostfd = -1;
         }
-        s->vhost_net = vhost_net_init(&s->nc, vhostfd);
+        s->vhost_net = vhost_net_init(&s->nc, vhostfd, force);
         if (!s->vhost_net) {
             error_report("vhost-net requested but could not be initialized");
             return -1;

qemu-options.hx +3 -1

index 898561d..11c93a2 100644
@@ -1050,7 +1050,7 @@ DEF("net", HAS_ARG, QEMU_OPTION_net,
     "-net tap[,vlan=n][,name=str],ifname=name\n"
     "                connect the host TAP network interface to VLAN 'n'\n"
 #else
-    "-net tap[,vlan=n][,name=str][,fd=h][,ifname=name][,script=file][,downscript=dfile][,sndbuf=nbytes][,vnet_hdr=on|off][,vhost=on|off][,vhostfd=h]\n"
+    "-net tap[,vlan=n][,name=str][,fd=h][,ifname=name][,script=file][,downscript=dfile][,sndbuf=nbytes][,vnet_hdr=on|off][,vhost=on|off][,vhostfd=h][,vhostforce=on|off]\n"
     "                connect the host TAP network interface to VLAN 'n' and use the\n"
     "                network scripts 'file' (default=" DEFAULT_NETWORK_SCRIPT ")\n"
     "                and 'dfile' (default=" DEFAULT_NETWORK_DOWN_SCRIPT ")\n"

@@ -1061,6 +1061,8 @@ DEF("net", HAS_ARG, QEMU_OPTION_net,
     "                use vnet_hdr=off to avoid enabling the IFF_VNET_HDR tap flag\n"
     "                use vnet_hdr=on to make the lack of IFF_VNET_HDR support an error condition\n"
     "                use vhost=on to enable experimental in kernel accelerator\n"
+    "                    (only has effect for virtio guests which use MSIX)\n"
+    "                use vhostforce=on to force vhost on for non-MSIX virtio guests\n"
     "                use 'vhostfd=h' to connect to an already opened vhost net device\n"
 #endif
     "-net socket[,vlan=n][,name=str][,fd=h][,listen=[host]:port][,connect=host:port]\n"

Quoted 36 lines · Michael S. Tsirkin

Quoted 29 lines · Alex Williamson

Quoted 11 lines · Michael S. Tsirkin

When MSI is off, each interrupt needs to be bounced through the io thread when it's set/cleared, so vhost-net causes more context switches and higher CPU utilization than userspace virtio which handles networking in the same thread.

We'll need to fix this by adding level irq support in kvm irqfd, for now disable vhost-net in these configurations.

Added a vhostforce flag to force vhost-net back on.

Signed-off-by Michael S. Tsirkin <[email protected]>

Quoted 8 lines

---

Untested, I'll send a pull request later after
some testing assuming we've resolved the
command line comments to everyone's satisfaction.

Changes since v1:
added vhostforce to enable vhost for non-MSI guests

I'm still not thrilled with the errno hack. If this is really a generic problem, we should better architect the set_guest_notifiers() callback, the force option feels like a band-aide.
I'm still curious if we can't
be more dynamic about how we setup the set_guest_notifiers() function pointer so it only gets set if (force || msix_enabled()) and we can limit some of the impact here to vhost. Thanks,

Quoted 1 line

Alex

I'm not entirely happy with the API either, but I'm sure playing with the function pointer is not the solution we are looking for:

Quoted 4 lines

msix is enabled/disabled dynamically. force is static but it is something net backend knows about, virtio pci doesn't. So I suspect modifying the callback on the fly will become messy quite quickly.

Isn't there a callback to the device when msix is enabled/disabled?

Quoted 2 lines

Well, this API will be easy to tweak after the fact, there's a single user after all.

Gosh, I wish I got to take that attitude when I submit code... ;)

Alex

Quoted 236 lines

Quoted 236 lines

Quoted 234 lines

 hw/vhost.c      |   16 +++++++++++-----
 hw/vhost.h      |    3 ++-
 hw/vhost_net.c  |    8 +++++---
 hw/vhost_net.h  |    2 +-
 hw/virtio-net.c |    6 ++++--
 hw/virtio-pci.c |    6 +++++-
 hw/virtio.h     |    4 +++-
 net/tap.c       |    6 ++++--
 qemu-options.hx |    4 +++-
 9 files changed, 38 insertions(+), 17 deletions(-)

hw/vhost.c +11 -5

index 6082da2..72df75f 100644
@@ -581,7 +581,7 @@ static void vhost_virtqueue_cleanup(struct vhost_dev *dev,
                               0, virtio_queue_get_desc_size(vdev, idx));
 }
 
-int vhost_dev_init(struct vhost_dev *hdev, int devfd)
+int vhost_dev_init(struct vhost_dev *hdev, int devfd, bool force)
 {
     uint64_t features;
     int r;

@@ -613,6 +613,7 @@ int vhost_dev_init(struct vhost_dev *hdev, int devfd)
     hdev->log_enabled = false;
     hdev->started = false;
     cpu_register_phys_memory_client(&hdev->client);
+    hdev->force = force;
     return 0;
 fail:
     r = -errno;

@@ -636,9 +637,12 @@ int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev)
         goto fail;
     }
 
-    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, true);
+    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, true,
+                                           hdev->force);
     if (r < 0) {
-        fprintf(stderr, "Error binding guest notifier: %d\n", -r);
+	if (r != -EVIRTIO_DISABLED) {
+		fprintf(stderr, "Error binding guest notifier: %d\n", -r);
+	}
         goto fail_notifiers;
     }
 

@@ -686,7 +690,8 @@ fail_vq:
     }
 fail_mem:
 fail_features:
-    vdev->binding->set_guest_notifiers(vdev->binding_opaque, false);
+    vdev->binding->set_guest_notifiers(vdev->binding_opaque, false,
+                                       hdev->force);
 fail_notifiers:
 fail:
     return r;

@@ -704,7 +709,8 @@ void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev)
     }
     vhost_client_sync_dirty_bitmap(&hdev->client, 0,
                                    (target_phys_addr_t)~0x0ull);
-    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, false);
+    r = vdev->binding->set_guest_notifiers(vdev->binding_opaque, false,
+                                           hdev->force);
     if (r < 0) {
         fprintf(stderr, "vhost guest notifier cleanup failed: %d\n", r);
         fflush(stderr);

hw/vhost.h +2 -1

index 86dd834..d5d4d86 100644
@@ -38,9 +38,10 @@ struct vhost_dev {
     bool log_enabled;
     vhost_log_chunk_t *log;
     unsigned long long log_size;
+    bool force;
 };
 
-int vhost_dev_init(struct vhost_dev *hdev, int devfd);
+int vhost_dev_init(struct vhost_dev *hdev, int devfd, bool force);
 void vhost_dev_cleanup(struct vhost_dev *hdev);
 int vhost_dev_start(struct vhost_dev *hdev, VirtIODevice *vdev);
 void vhost_dev_stop(struct vhost_dev *hdev, VirtIODevice *vdev);

hw/vhost_net.c +5 -3

index c068be1..4f57271 100644
@@ -81,7 +81,8 @@ static int vhost_net_get_fd(VLANClientState *backend)
     }
 }
 
-struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
+struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd,
+                                 bool force)
 {
     int r;
     struct vhost_net *net = qemu_malloc(sizeof *net);

@@ -98,7 +99,7 @@ struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
         (1 << VHOST_NET_F_VIRTIO_NET_HDR);
     net->backend = r;
 
-    r = vhost_dev_init(&net->dev, devfd);
+    r = vhost_dev_init(&net->dev, devfd, force);
     if (r < 0) {
         goto fail;
     }

@@ -188,7 +189,8 @@ void vhost_net_cleanup(struct vhost_net *net)
     qemu_free(net);
 }
 #else
-struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd)
+struct vhost_net *vhost_net_init(VLANClientState *backend, int devfd,
+                                 bool force)
 {
 	return NULL;
 }

hw/vhost_net.h +1 -1

index 6c18ff7..8435094 100644
@@ -6,7 +6,7 @@
 struct vhost_net;
 typedef struct vhost_net VHostNetState;
 
-VHostNetState *vhost_net_init(VLANClientState *backend, int devfd);
+VHostNetState *vhost_net_init(VLANClientState *backend, int devfd, bool force);
 
 int vhost_net_start(VHostNetState *net, VirtIODevice *dev);
 void vhost_net_stop(VHostNetState *net, VirtIODevice *dev);

hw/virtio-net.c +4 -2

index ccb3e63..5de3fee 100644
@@ -121,8 +121,10 @@ static void virtio_net_vhost_status(VirtIONet *n, uint8_t status)
     if (!n->vhost_started) {
         int r = vhost_net_start(tap_get_vhost_net(n->nic->nc.peer), &n->vdev);
         if (r < 0) {
-            error_report("unable to start vhost net: %d: "
-                         "falling back on userspace virtio", -r);
+            if (r != -EVIRTIO_DISABLED) {
+                error_report("unable to start vhost net: %d: "
+                             "falling back on userspace virtio", -r);
+            }
         } else {
             n->vhost_started = 1;
         }

hw/virtio-pci.c +5 -1

index d07ff97..c93ef11 100644
@@ -595,12 +595,16 @@ static int virtio_pci_set_guest_notifier(void *opaque, int n, bool assign)
     return 0;
 }
 
-static int virtio_pci_set_guest_notifiers(void *opaque, bool assign)
+static int virtio_pci_set_guest_notifiers(void *opaque, bool assign, bool force)
 {
     VirtIOPCIProxy *proxy = opaque;
     VirtIODevice *vdev = proxy->vdev;
     int r, n;
 
+    if (assign && !force && !msix_enabled(&proxy->pci_dev)) {
+	    return -EVIRTIO_DISABLED;
+    }
+
     for (n = 0; n < VIRTIO_PCI_QUEUE_MAX; n++) {
         if (!virtio_queue_get_num(vdev, n)) {
             break;

hw/virtio.h +3 -1

index d8546d5..156c9a3 100644
@@ -93,11 +93,13 @@ typedef struct {
     int (*load_config)(void * opaque, QEMUFile *f);
     int (*load_queue)(void * opaque, int n, QEMUFile *f);
     unsigned (*get_features)(void * opaque);
-    int (*set_guest_notifiers)(void * opaque, bool assigned);
+    int (*set_guest_notifiers)(void * opaque, bool assigned, bool force);
     int (*set_host_notifier)(void * opaque, int n, bool assigned);
     void (*vmstate_change)(void * opaque, bool running);
 } VirtIOBindings;
 
+#define EVIRTIO_DISABLED ERANGE
+
 #define VIRTIO_PCI_QUEUE_MAX 64
 
 #define VIRTIO_NO_VECTOR 0xffff

net/tap.c +4 -2

index eada34a..b8cd252 100644
@@ -491,8 +491,10 @@ int net_init_tap(QemuOpts *opts, Monitor *mon, const char *name, VLANState *vlan
         }
     }
 
-    if (qemu_opt_get_bool(opts, "vhost", !!qemu_opt_get(opts, "vhostfd"))) {
+    if (qemu_opt_get_bool(opts, "vhost", !!qemu_opt_get(opts, "vhostfd") ||
+                          qemu_opt_get_bool(opts, "vhostforce", false))) {
         int vhostfd, r;
+        bool force = qemu_opt_get_bool(opts, "vhostforce", false);
         if (qemu_opt_get(opts, "vhostfd")) {
             r = net_handle_fd_param(mon, qemu_opt_get(opts, "vhostfd"));
             if (r == -1) {

@@ -502,7 +504,7 @@ int net_init_tap(QemuOpts *opts, Monitor *mon, const char *name, VLANState *vlan
         } else {
             vhostfd = -1;
         }
-        s->vhost_net = vhost_net_init(&s->nc, vhostfd);
+        s->vhost_net = vhost_net_init(&s->nc, vhostfd, force);
         if (!s->vhost_net) {
             error_report("vhost-net requested but could not be initialized");
             return -1;

qemu-options.hx +3 -1

index 898561d..11c93a2 100644
@@ -1050,7 +1050,7 @@ DEF("net", HAS_ARG, QEMU_OPTION_net,
     "-net tap[,vlan=n][,name=str],ifname=name\n"
     "                connect the host TAP network interface to VLAN 'n'\n"
 #else
-    "-net tap[,vlan=n][,name=str][,fd=h][,ifname=name][,script=file][,downscript=dfile][,sndbuf=nbytes][,vnet_hdr=on|off][,vhost=on|off][,vhostfd=h]\n"
+    "-net tap[,vlan=n][,name=str][,fd=h][,ifname=name][,script=file][,downscript=dfile][,sndbuf=nbytes][,vnet_hdr=on|off][,vhost=on|off][,vhostfd=h][,vhostforce=on|off]\n"
     "                connect the host TAP network interface to VLAN 'n' and use the\n"
     "                network scripts 'file' (default=" DEFAULT_NETWORK_SCRIPT ")\n"
     "                and 'dfile' (default=" DEFAULT_NETWORK_DOWN_SCRIPT ")\n"

@@ -1061,6 +1061,8 @@ DEF("net", HAS_ARG, QEMU_OPTION_net,
     "                use vnet_hdr=off to avoid enabling the IFF_VNET_HDR tap flag\n"
     "                use vnet_hdr=on to make the lack of IFF_VNET_HDR support an error condition\n"
     "                use vhost=on to enable experimental in kernel accelerator\n"
+    "                    (only has effect for virtio guests which use MSIX)\n"
+    "                use vhostforce=on to force vhost on for non-MSIX virtio guests\n"
     "                use 'vhostfd=h' to connect to an already opened vhost net device\n"
 #endif
     "-net socket[,vlan=n][,name=str][,fd=h][,listen=[host]:port][,connect=host:port]\n"