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Date:   Tue, 5 May 2020 11:22:13 +1000
From:   Gavin Shan <>
To:     Vitaly Kuznetsov <>,,
Cc:, Paolo Bonzini <>,
        Andy Lutomirski <>,
        Thomas Gleixner <>,
        Ingo Molnar <>, Borislav Petkov <>,
        "H. Peter Anvin" <>,
        Sean Christopherson <>,
        Wanpeng Li <>,
        Jim Mattson <>
Subject: Re: [PATCH RFC 1/6] Revert "KVM: async_pf: Fix #DF due to inject
 "Page not Present" and "Page Ready" exceptions simultaneously"

On 4/29/20 7:36 PM, Vitaly Kuznetsov wrote:
> Commit 9a6e7c39810e (""KVM: async_pf: Fix #DF due to inject "Page not
> Present" and "Page Ready" exceptions simultaneously") added a protection
> against 'page ready' notification coming before 'page not ready' is
> delivered. This situation seems to be impossible since commit 2a266f23550b
> ("KVM MMU: check pending exception before injecting APF) which added
> 'vcpu->arch.exception.pending' check to kvm_can_do_async_pf.
> On x86, kvm_arch_async_page_present() has only one call site:
> kvm_check_async_pf_completion() loop and we only enter the loop when
> kvm_arch_can_inject_async_page_present(vcpu) which when async pf msr
> is enabled, translates into kvm_can_do_async_pf().
> There is also one problem with the cancellation mechanism. We don't seem
> to check that the 'page not ready' notification we're cancelling matches
> the 'page ready' notification so in theory, we may erroneously drop two
> valid events.
> Revert the commit. apf_get_user() stays as we will need it for the new
> 'page ready notifications via interrupt' mechanism.
> Signed-off-by: Vitaly Kuznetsov <>
> ---
>   arch/x86/kvm/x86.c | 16 +---------------
>   1 file changed, 1 insertion(+), 15 deletions(-)

Reviewed-by: Gavin Shan <>

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