KVM: x86: fix backward migration with async_PF

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.

To avoid breaking different cases, we are checking for CPUID feature bit
before enabling the feature and nothing else.

Fixes: 52a5c155cf79 ("KVM: async_pf: Let guest support delivery of async_pf from guest mode")
Cc: <stable@vger.kernel.org>
Reviewed-by: Wanpeng Li <wanpengli@tencent.com>
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
index dcab6dc..87a7506 100644 (file)
--- a/Documentation/virtual/kvm/cpuid.txt
+++ b/Documentation/virtual/kvm/cpuid.txt
@@ -58,6 +58,10 @@ KVM_FEATURE_PV_TLB_FLUSH           ||     9 || guest checks this feature bit
                                    ||       || before enabling paravirtualized
                                    ||       || tlb flush.
 ------------------------------------------------------------------------------
+KVM_FEATURE_ASYNC_PF_VMEXIT        ||    10 || paravirtualized async PF VM exit
+                                   ||       || can be enabled by setting bit 2
+                                   ||       || when writing to msr 0x4b564d02
+------------------------------------------------------------------------------
 KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no guest-side
                                    ||       || per-cpu warps are expected in
                                    ||       || kvmclock.

diff --git a/Documentation/virtual/kvm/msr.txt b/Documentation/virtual/kvm/msr.txt
index 1ebecc1..f3f0d57 100644 (file)
--- a/Documentation/virtual/kvm/msr.txt
+++ b/Documentation/virtual/kvm/msr.txt
@@ -170,7 +170,8 @@ MSR_KVM_ASYNC_PF_EN: 0x4b564d02
        when asynchronous page faults are enabled on the vcpu 0 when
        disabled. Bit 1 is 1 if asynchronous page faults can be injected
        when vcpu is in cpl == 0. Bit 2 is 1 if asynchronous page faults
-       are delivered to L1 as #PF vmexits.
+       are delivered to L1 as #PF vmexits.  Bit 2 can be set only if
+       KVM_FEATURE_ASYNC_PF_VMEXIT is present in CPUID.
 
        First 4 byte of 64 byte memory location will be written to by
        the hypervisor at the time of asynchronous page fault (APF)

diff --git a/arch/x86/include/uapi/asm/kvm_para.h b/arch/x86/include/uapi/asm/kvm_para.h
index 7a2ade4..6cfa9c8 100644 (file)
--- 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 4e37d1a..971babe 100644 (file)
--- 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 a0c5a69..b671fc2 100644 (file)
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -607,7 +607,8 @@ 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);