Xen PV-on-HVM guest support (v3)

Support for Xen PV-on-HVM guests can be implemented almost entirely in userspace, except for handling one annoying MSR that maps a Xen hypercall blob into guest address space.

A generic mechanism to delegate MSR writes to userspace seems overkill and risks encouraging similar MSR abuse in the future. Thus this patch adds special support for the Xen HVM MSR.

I implemented a new ioctl, KVM_XEN_HVM_CONFIG, that lets userspace tell KVM which MSR the guest will write to, as well as the starting address and size of the hypercall blobs (one each for 32-bit and 64-bit) that userspace has loaded from files. When the guest writes to the MSR, KVM copies one page of the blob from userspace to the guest.

I've tested this patch with a hacked-up version of Gerd's userspace code, booting a number of guests (CentOS 5.3 i386 and x86_64, and FreeBSD 8.0-RC1 amd64) and exercising PV network and block devices.

v3: separate blob_{addr,size}_{32,64}; move xen_hvm_config to struct kvm_arch; remove unneeded ifdefs; return -EFAULT, -E2BIG, etc. from xen_hvm_config; use is_long_mode(); remove debug printks; document ioctl in api.txt

---

index 5a4bc8c..5980113 100644
@@ -593,6 +593,30 @@ struct kvm_irqchip {
 	} chip;
 };
 
+4.27 KVM_XEN_HVM_CONFIG
+
+Capability: KVM_CAP_XEN_HVM
+Architectures: x86
+Type: vm ioctl
+Parameters: struct kvm_xen_hvm_config (in)
+Returns: 0 on success, -1 on error
+
+Sets the MSR that the Xen HVM guest uses to initialize its hypercall
+page, and provides the starting address and size of the hypercall
+blobs in userspace.  When the guest writes the MSR, kvm copies one
+page of a blob (32- or 64-bit, depending on the vcpu mode) to guest
+memory.
+
+struct kvm_xen_hvm_config {
+	__u32 msr;
+	__u32 pad1;
+	__u64 blob_addr_32;
+	__u64 blob_addr_64;
+	__u8 blob_size_32;
+	__u8 blob_size_64;
+	__u8 pad2[30];
+};
+
 5. The kvm_run structure
 
 Application code obtains a pointer to the kvm_run structure by

index f02e87a..ef9b4b7 100644
@@ -19,6 +19,7 @@
 #define __KVM_HAVE_MSIX
 #define __KVM_HAVE_MCE
 #define __KVM_HAVE_PIT_STATE2
+#define __KVM_HAVE_XEN_HVM
 
 /* Architectural interrupt line count. */
 #define KVM_NR_INTERRUPTS 256

b/arch/x86/include/asm/kvm_host.h
index 45226f0..aee95b2 100644

@@ -410,6 +410,8 @@ struct kvm_arch{
 
 	unsigned long irq_sources_bitmap;
 	u64 vm_init_tsc;
+
+	struct kvm_xen_hvm_config xen_hvm_config;
 };
 
 struct kvm_vm_stat {

index 1d454d9..66149fa 100644
@@ -835,6 +835,37 @@ static int set_msr_mce(struct kvm_vcpu *vcpu, u32

msr, u64 data)
return 0;
}

+static int xen_hvm_config(struct kvm_vcpu *vcpu, u64 data)
+{
+ int lm = is_long_mode(vcpu);
+ u8 *blob_addr = lm ? (u8 *)vcpu->kvm->arch.xen_hvm_config.blob_addr_64
+ : (u8 *)vcpu->kvm->arch.xen_hvm_config.blob_addr_32;
+ u8 blob_size = lm ? vcpu->kvm->arch.xen_hvm_config.blob_size_64
+ : vcpu->kvm->arch.xen_hvm_config.blob_size_32;
+ u32 page_num = data & ~PAGE_MASK;
+ u64 page_addr = data & PAGE_MASK;
+ u8 *page;
+ int r;
+
+ r = -E2BIG;
+ if (page_num >= blob_size)
+ goto out;
+ r = -ENOMEM;
+ page = kzalloc(PAGE_SIZE, GFP_KERNEL);
+ if (!page)
+ goto out;
+ r = -EFAULT;
+ if (copy_from_user(page, blob_addr + (page_num * PAGE_SIZE),
PAGE_SIZE))
+ goto out_free;
+ if (kvm_write_guest(vcpu->kvm, page_addr, page, PAGE_SIZE))
+ goto out_free;
+ r = 0;
+out_free:
+ kfree(page);
+out:
+ return r;
+}
+
int kvm_set_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 data) {
switch (msr) {
@@ -950,6 +981,8 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 data)
"0x%x data 0x%llx\n", msr, data);
break;
default:
+ if (msr && (msr == vcpu->kvm->arch.xen_hvm_config.msr))
+ return xen_hvm_config(vcpu, data);
if (!ignore_msrs) {
pr_unimpl(vcpu, "unhandled wrmsr: 0x%x data %llx\n",
msr, data);
@@ -2411,6 +2444,14 @@ long kvm_arch_vm_ioctl(struct file *filp,
r = 0;
break;
}
+ case KVM_XEN_HVM_CONFIG: {
+ r = -EFAULT;
+ if (copy_from_user(&kvm->arch.xen_hvm_config, argp,
+ sizeof(struct kvm_xen_hvm_config)))
+ goto out;
+ r = 0;
+ break;
+ }
default:
;
}

index f8f8900..e0ed46c 100644
@@ -436,6 +436,9 @@ struct kvm_ioeventfd {
 #endif
 #define KVM_CAP_IOEVENTFD 36
 #define KVM_CAP_SET_IDENTITY_MAP_ADDR 37
+#ifdef __KVM_HAVE_XEN_HVM
+#define KVM_CAP_XEN_HVM 38
+#endif
 
 #ifdef KVM_CAP_IRQ_ROUTING
 

@@ -488,6 +491,18 @@ struct kvm_x86_mce {
 };
 #endif
 
+#ifdef KVM_CAP_XEN_HVM
+struct kvm_xen_hvm_config {
+	__u32 msr;
+	__u32 pad1;
+	__u64 blob_addr_32;
+	__u64 blob_addr_64;
+	__u8 blob_size_32;
+	__u8 blob_size_64;
+	__u8 pad2[30];
+};
+#endif
+
 #define KVM_IRQFD_FLAG_DEASSIGN (1 << 0)
 
 struct kvm_irqfd {

@@ -546,6 +561,7 @@ struct kvm_irqfd {
 #define KVM_CREATE_PIT2		   _IOW(KVMIO, 0x77, struct kvm_pit_config)
 #define KVM_SET_BOOT_CPU_ID        _IO(KVMIO, 0x78)
 #define KVM_IOEVENTFD             _IOW(KVMIO, 0x79, struct

kvm_ioeventfd)
+#define KVM_XEN_HVM_CONFIG _IOW(KVMIO, 0x7a, struct
kvm_xen_hvm_config)

/*
* ioctls for vcpu fds