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From:   Wanpeng Li <kernellwp@gmail.com>
Date:   Fri, 2 Feb 2018 09:44:55 +0800
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Subject: Re: [PATCH v2] KVM: x86: fix backward migration with async_PF
To:     =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>
Cc:     linux-kernel@vger.kernel.org, kvm <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Wanpeng Li <wanpeng.li@hotmail.com>,
        "# v3 . 10+" <stable@vger.kernel.org>
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2018-02-02 5:16 GMT+08:00 Radim Kr=C4=8Dm=C3=A1=C5=99 <rkrcmar@redhat.com>:
> 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>
> Signed-off-by: Radim Kr=C4=8Dm=C3=A1=C5=99 <rkrcmar@redhat.com>

Reviewed-by: Wanpeng Li <wanpengli@tencent.com>

> ---
> v2:
> * added documentation [Paolo]
> * retained compatibility recent kernels [Paolo]
> ---
>  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 dcab6dc11e3b..87a7506f31c2 100644
> --- a/Documentation/virtual/kvm/cpuid.txt
> +++ b/Documentation/virtual/kvm/cpuid.txt
> @@ -58,6 +58,10 @@ KVM_FEATURE_PV_TLB_FLUSH           ||     9 || guest c=
hecks this feature bit
>                                     ||       || before enabling paravirtu=
alized
>                                     ||       || tlb flush.
>  ------------------------------------------------------------------------=
------
> +KVM_FEATURE_ASYNC_PF_VMEXIT        ||    10 || paravirtualized async PF =
VM exit
> +                                   ||       || can be enabled by setting=
 bit 2
> +                                   ||       || when writing to msr 0x4b5=
64d02
> +------------------------------------------------------------------------=
------
>  KVM_FEATURE_CLOCKSOURCE_STABLE_BIT ||    24 || host will warn if no gues=
t-side
>                                     ||       || per-cpu warps are expecte=
d in
>                                     ||       || kvmclock.
> diff --git a/Documentation/virtual/kvm/msr.txt b/Documentation/virtual/kv=
m/msr.txt
> index 1ebecc115dc6..f3f0d57ced8e 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 =3D=3D 0. Bit 2 is 1 if asynchronous page fau=
lts
> -       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 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 |=3D 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) & 0xfff=
fffff, pa >> 32) < 0)
> -                       wrmsrl(MSR_KVM_ASYNC_PF_EN, pa);
> +               if (kvm_para_has_feature(KVM_FEATURE_ASYNC_PF_VMEXIT))
> +                       pa |=3D 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_ent=
ry2 *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 |=3D (1 << KVM_FEATURE_STEAL_TIME);
> --
> 2.15.1
>