Re: [PATCH] KVM: x86: check for interrupts before deciding whether to exit the fast path

From: Sean Christopherson <seanjc@google.com>
To: Paolo Bonzini <pbonzini@redhat.com>
Cc: linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	wanpengli@tencent.com, stable@vger.kernel.org
Subject: Re: [PATCH] KVM: x86: check for interrupts before deciding whether to exit the fast path
Date: Wed, 20 Oct 2021 19:42:05 +0000	[thread overview]
Message-ID: <YXBxDYJcrx/C9QDS@google.com> (raw)
In-Reply-To: <20211020145231.871299-1-pbonzini@redhat.com>

On Wed, Oct 20, 2021, Paolo Bonzini wrote:
> The kvm_x86_sync_pir_to_irr callback can sometimes set KVM_REQ_EVENT.
> If that happens exactly at the time that an exit is handled as
> EXIT_FASTPATH_REENTER_GUEST, vcpu_enter_guest will go incorrectly
> through the loop that calls kvm_x86_run, instead of processing
> the request promptly.
> 
> Fixes: 379a3c8ee444 ("KVM: VMX: Optimize posted-interrupt delivery for timer fastpath")
> Cc: stable@vger.kernel.org
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> ---

Reviewed-by: Sean Christopherson <seanjc@google.com>

>  arch/x86/kvm/x86.c | 10 +++++-----
>  1 file changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index fa48948b4934..b9b31e5f72b0 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9781,14 +9781,14 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
>  		if (likely(exit_fastpath != EXIT_FASTPATH_REENTER_GUEST))
>  			break;
>  
> -                if (unlikely(kvm_vcpu_exit_request(vcpu))) {
> +		if (vcpu->arch.apicv_active)
> +			static_call(kvm_x86_sync_pir_to_irr)(vcpu);
> +
> +		if (unlikely(kvm_vcpu_exit_request(vcpu))) {
>  			exit_fastpath = EXIT_FASTPATH_EXIT_HANDLED;
>  			break;
>  		}
> -
> -		if (vcpu->arch.apicv_active)
> -			static_call(kvm_x86_sync_pir_to_irr)(vcpu);
> -        }
> +	}

I think someone working on git has a meta-entry in the obfuscated C context.
This is the most convoluted diff possible for a simple code move :-)

>  	/*
>  	 * Do this here before restoring debug registers on the host.  And
> -- 
> 2.27.0
> 