[PATCH] KVM: x86: fix backward migration with async_PF

[Radim KrÄmÃÅ]

Guests on new hypersiors might set KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT
bit when enabling async_PF, but this bit is reserved on old hypervisors,
which results in a failure upon migration.

Guests at least expect that KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT might not
be present when booting, so we allow userspace to handle migration
compatibility by adding a KVM CPUID flag that determines the presence of
KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT.

Fixes: 52a5c155cf79 ("KVM: async_pf: Let guest support delivery of async_pf from guest mode")
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
---
 arch/x86/include/uapi/asm/kvm_para.h |  1 +
 arch/x86/kernel/kvm.c                |  8 ++++----
 arch/x86/kvm/cpuid.c                 |  3 ++-
 arch/x86/kvm/cpuid.h                 | 11 +++++++++++
 arch/x86/kvm/x86.c                   |  6 ++++--
 5 files changed, 22 insertions(+), 7 deletions(-)

diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
index 7a2ade4aa235..6cfa9c8cb7d6 100644
--- a/arch/x86/include/uapi/asm/kvm_para.h
+++ b/arch/x86/include/uapi/asm/kvm_para.h
@@ -26,6 +26,7 @@
 #define KVM_FEATURE_PV_EOI		6
 #define KVM_FEATURE_PV_UNHALT		7
 #define KVM_FEATURE_PV_TLB_FLUSH	9
+#define KVM_FEATURE_ASYNC_PF_VMEXIT	10
 
 /* The last 8 bits are used to indicate how to interpret the flags field
  * in pvclock structure. If no bits are set, all flags are ignored.
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index 4e37d1a851a6..971babe964d2 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -341,10 +341,10 @@ static void kvm_guest_cpu_init(void)
 #endif
 		pa |= KVM_ASYNC_PF_ENABLED;
 
-		/* Async page fault support for L1 hypervisor is optional */
-		if (wrmsr_safe(MSR_KVM_ASYNC_PF_EN,
-			(pa | KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT) & 0xffffffff, pa >> 32) < 0)
-			wrmsrl(MSR_KVM_ASYNC_PF_EN, pa);
+		if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF_VMEXIT))
+			pa |= KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT;
+
+		wrmsrl(MSR_KVM_ASYNC_PF_EN, pa);
 		__this_cpu_write(apf_reason.enabled, 1);
 		printk(KERN_INFO"KVM setup async PF for cpu %d\n",
 		       smp_processor_id());
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 20e491b94f44..7fc04a176c57 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -604,7 +604,8 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
 			     (1 << KVM_FEATURE_PV_EOI) |
 			     (1 << KVM_FEATURE_CLOCKSOURCE_STABLE_BIT) |
 			     (1 << KVM_FEATURE_PV_UNHALT) |
-			     (1 << KVM_FEATURE_PV_TLB_FLUSH);
+			     (1 << KVM_FEATURE_PV_TLB_FLUSH) |
+			     (1 << KVM_FEATURE_ASYNC_PF_VMEXIT);
 
 		if (sched_info_on())
 			entry->eax |= (1 << KVM_FEATURE_STEAL_TIME);
diff --git a/arch/x86/kvm/cpuid.h b/arch/x86/kvm/cpuid.h
index c2cea6651279..f20731dfe28e 100644
--- a/arch/x86/kvm/cpuid.h
+++ b/arch/x86/kvm/cpuid.h
@@ -105,6 +105,17 @@ static __always_inline bool guest_cpuid_has(struct kvm_vcpu *vcpu, unsigned x86_
 	return *reg & bit(x86_feature);
 }
 
+static inline bool guest_kvm_cpuid_has(struct kvm_vcpu *vcpu, unsigned kvm_feature)
+{
+	struct kvm_cpuid_entry2 *entry;
+
+	entry = kvm_find_cpuid_entry(vcpu, KVM_CPUID_FEATURES, 0);
+	if (!entry)
+		return false;
+
+	return entry->eax & bit(kvm_feature);
+}
+
 static __always_inline void guest_cpuid_clear(struct kvm_vcpu *vcpu, unsigned x86_feature)
 {
 	int *reg;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 4c3103f449a3..c16740a06f0c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2139,8 +2139,10 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data)
 {
 	gpa_t gpa = data & ~0x3f;
 
-	/* Bits 3:5 are reserved, Should be zero */
-	if (data & 0x38)
+	/* Bits 3:5 are reserved, Should be zero. */
+	if (data & 0x38 ||
+	    (data & KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT &&
+	     !guest_kvm_cpuid_has(vcpu, KVM_FEATURE_ASYNC_PF_VMEXIT)))
 		return 1;
 
 	vcpu->arch.apf.msr_val = data;
-- 
2.15.1