[PATCH] Xen PV-on-HVM guest support (v2)

[Ed Swierk <eswierk@aristanetworks.com>]

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.

v2: fix ioctl struct padding; renumber CAP and ioctl constants; check
kvm_write_guest() return value; change printks to KERN_DEBUG (I think
they're worth keeping for debugging userspace)

Signed-off-by: Ed Swierk <eswierk@aristanetworks.com>

---
Index: kvm-kmod/include/asm-x86/kvm.h
===================================================================
--- kvm-kmod.orig/include/asm-x86/kvm.h
+++ kvm-kmod/include/asm-x86/kvm.h
@@ -59,6 +59,7 @@
 #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
Index: kvm-kmod/include/linux/kvm.h
===================================================================
--- kvm-kmod.orig/include/linux/kvm.h
+++ kvm-kmod/include/linux/kvm.h
@@ -476,6 +476,9 @@ 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
 
@@ -528,6 +531,15 @@ struct kvm_x86_mce {
 };
 #endif
 
+#ifdef KVM_CAP_XEN_HVM
+struct kvm_xen_hvm_config {
+	__u32 msr;
+	__u8 pad[2];
+	__u8 blob_size[2];
+	__u64 blob_addr[2];
+};
+#endif
+
 #define KVM_IRQFD_FLAG_DEASSIGN (1 << 0)
 
 struct kvm_irqfd {
@@ -586,6 +598,7 @@ 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
Index: kvm-kmod/include/linux/kvm_host.h
===================================================================
--- kvm-kmod.orig/include/linux/kvm_host.h
+++ kvm-kmod/include/linux/kvm_host.h
@@ -236,6 +236,10 @@ struct kvm {
 	unsigned long mmu_notifier_seq;
 	long mmu_notifier_count;
 #endif
+
+#ifdef KVM_CAP_XEN_HVM
+	struct kvm_xen_hvm_config xen_hvm_config;
+#endif
 };
 
 /* The guest did something we don't support. */
Index: kvm-kmod/x86/x86.c
===================================================================
--- kvm-kmod.orig/x86/x86.c
+++ kvm-kmod/x86/x86.c
@@ -875,6 +875,35 @@ static int set_msr_mce(struct kvm_vcpu *
 	return 0;
 }
 
+#ifdef KVM_CAP_XEN_HVM
+static int xen_hvm_config(struct kvm_vcpu *vcpu, u64 data)
+{
+	int blob = !!(vcpu->arch.shadow_efer & EFER_LME);
+	u32 pnum = data & ~PAGE_MASK;
+	u64 paddr = data & PAGE_MASK;
+	u8 *page;
+	int r = 1;
+
+	if (pnum >= vcpu->kvm->xen_hvm_config.blob_size[blob])
+		goto out;
+	page = kzalloc(PAGE_SIZE, GFP_KERNEL);
+	if (!page)
+		goto out;
+	if (copy_from_user(page, (u8 *)vcpu->kvm->xen_hvm_config.blob_addr[blob]
+			   + pnum * PAGE_SIZE, PAGE_SIZE))
+		goto out_free;
+	if (kvm_write_guest(vcpu->kvm, paddr, page, PAGE_SIZE))
+		goto out_free;
+	printk(KERN_DEBUG "kvm: copied xen hvm blob %d page %d to 0x%llx\n",
+	       blob, pnum, paddr);
+	r = 0;
+out_free:
+	kfree(page);
+out:
+	return r;
+}
+#endif
+
 int kvm_set_msr_common(struct kvm_vcpu *vcpu, u32 msr, u64 data)
 {
 	switch (msr) {
@@ -990,6 +1019,10 @@ int kvm_set_msr_common(struct kvm_vcpu *
 			"0x%x data 0x%llx\n", msr, data);
 		break;
 	default:
+#ifdef KVM_CAP_XEN_HVM
+		if (msr && (msr == vcpu->kvm->xen_hvm_config.msr))
+			return xen_hvm_config(vcpu, data);
+#endif
 		if (!ignore_msrs) {
 			pr_unimpl(vcpu, "unhandled wrmsr: 0x%x data %llx\n",
 				msr, data);
@@ -2453,6 +2486,17 @@ long kvm_arch_vm_ioctl(struct file *filp
 		r = 0;
 		break;
 	}
+#ifdef KVM_CAP_XEN_HVM
+	case KVM_XEN_HVM_CONFIG: {
+		r = -EFAULT;
+		if (copy_from_user(&kvm->xen_hvm_config, argp,
+				   sizeof(struct kvm_xen_hvm_config)))
+			goto out;
+		printk(KERN_DEBUG "kvm: configured xen hvm\n");
+		r = 0;
+		break;
+	}
+#endif
 	default:
 		;
 	}