tap: reset vnet header size on open

PATCH Superseded Michael S. Tsirkin 2 participants 2 messages Nov 12, 2012 Networking

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Michael S. Tsirkin

author

2012-11-12 07:16 UTC #0 lore raw

PATCH 1/1 tap: reset vnet header size on open

For tap, we currently assume the vnet header size is 10 (the default value) but that might not be the case if tap is persistent and has been used by qemu previously. To fix, set vnet header size correctly on open.

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

---

This fixes the issue reported by Alexander Graf on the kvm forum. Alexander, could you confirm please?
Thanks,

 net/tap-linux.c | 8 ++++++++
 1 file changed, 8 insertions(+)

net/tap-linux.c +8 -0

index c6521be..3eaedc4 100644
@@ -39,6 +39,7 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr, int vnet_hdr_required
 {
     struct ifreq ifr;
     int fd, ret;
+    int len = sizeof(struct virtio_net_hdr);
 
     TFR(fd = open(PATH_NET_TUN, O_RDWR));
     if (fd < 0) {

@@ -65,6 +66,13 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr, int vnet_hdr_required
             close(fd);
             return -1;
         }
+        /*
+         * Make sure vnet header size has the default value: for a persistent
+         * tap it might have been modified e.g. by another instance of qemu.
+         * Ignore errors since old kernels do not support this ioctl: in this
+         * case the header size implicitly has the correct value.
+         */
+        ioctl(fd, TUNSETVNETHDRSZ, &len);
     }
 
     if (ifname[0] != '\0')

Signature · 1 line

MST

Stefan Hajnoczi

2012-11-12 12:23 UTC #1 lore raw

Quoted 20 lines · Michael S. Tsirkin

For tap, we currently assume the vnet header size is 10 (the default value) but that might not be the case if tap is persistent and has been used by qemu previously. To fix, set vnet header size correctly on open.
Signed-off-by Michael S. Tsirkin <[email protected]>
---
This fixes the issue reported by Alexander Graf on the kvm forum. Alexander, could you confirm please?
Thanks,
 net/tap-linux.c | 8 ++++++++
 1 file changed, 8 insertions(+)
net/tap-linux.c +0 -0
index c6521be..3eaedc4 100644

net/tap-linux.c int tap_open(char *ifname, int ifname_size, int *vnet_hdr, int vnet_hdr_required · line 42

Quoted 4 lines

net/tap-linux.c +1 -0

@@ -39,6 +39,7 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr, int vnet_hdr_required
 {
     struct ifreq ifr;
     int fd, ret;
+    int len = sizeof(struct virtio_net_hdr);

We probably want the same behavior in the tap,fd= codepath. How about a call to tap_set_vnet_hdr_len() in net_init_tap() instead?

Stefan