Re: [stable-4.14/4.15] KVM: x86: fix backward migration with async_PF

[Jinpu Wang <jinpu.wang@profitbricks.com>]

On Fri, Mar 9, 2018 at 3:43 PM, Paolo Bonzini <pbonzini@redhat.com> wrote:
> On 09/03/2018 14:20, Jack Wang wrote:
>> From: Radim Krčmář <rkrcmar@redhat.com>
>>
>> commit fe2a3027e74e40a3ece3a4c1e4e51403090a907a upstream.
>>
>> 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>
>> [jwang: port to 4.14]
>> Signed-off-by: Jack Wang <jinpu.wang@profitbricks.com>
>> ---
>>  Documentation/virtual/kvm/cpuid.txt  | 4 ++++
>>  Documentation/virtual/kvm/msr.txt    | 3 ++-
>>  arch/x86/include/uapi/asm/kvm_para.h | 1 +
>>  arch/x86/kernel/kvm.c                | 8 ++++----
>>  arch/x86/kvm/cpuid.c                 | 3 ++-
>>  5 files changed, 13 insertions(+), 6 deletions(-)
>>
>> diff --git a/Documentation/virtual/kvm/cpuid.txt b/Documentation/virtual/kvm/cpuid.txt
>> index 3c65feb..a81c97a 100644
>> --- a/Documentation/virtual/kvm/cpuid.txt
>> +++ b/Documentation/virtual/kvm/cpuid.txt
>> @@ -54,6 +54,10 @@ KVM_FEATURE_PV_UNHALT              ||     7 || guest checks this feature bit
>>                                     ||       || before enabling paravirtualized
>>                                     ||       || spinlock support.
>>  ------------------------------------------------------------------------------
>> +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
>> --- 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 09cc064..989db88 100644
>> --- a/arch/x86/include/uapi/asm/kvm_para.h
>> +++ b/arch/x86/include/uapi/asm/kvm_para.h
>> @@ -25,6 +25,7 @@
>>  #define KVM_FEATURE_STEAL_TIME               5
>>  #define KVM_FEATURE_PV_EOI           6
>>  #define KVM_FEATURE_PV_UNHALT                7
>> +#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 b40ffbf..0a93e83b 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 13f5d42..4f544f2 100644
>> --- a/arch/x86/kvm/cpuid.c
>> +++ b/arch/x86/kvm/cpuid.c
>> @@ -597,7 +597,8 @@ static inline int __do_cpuid_ent(struct kvm_cpuid_entry2 *entry, u32 function,
>>                            (1 << KVM_FEATURE_ASYNC_PF) |
>>                            (1 << KVM_FEATURE_PV_EOI) |
>>                            (1 << KVM_FEATURE_CLOCKSOURCE_STABLE_BIT) |
>> -                          (1 << KVM_FEATURE_PV_UNHALT);
>> +                          (1 << KVM_FEATURE_PV_UNHALT) |
>> +                          (1 << KVM_FEATURE_ASYNC_PF_VMEXIT);
>>
>>               if (sched_info_on())
>>                       entry->eax |= (1 << KVM_FEATURE_STEAL_TIME);
>>
>
> Acked-by: Paolo Bonzini <pbonzini@redhat.com>
>
> Thanks Jack!
>
> Paolo

Thanks Paolo!

Jack