linux-kernel - Re: [PATCH 2/3] KVM: x86: Fix a semi theoretical bug in kvm_arch_async_page_present

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Message-ID: <aNLtMC-k95pIYfeq@google.com>
Date: Tue, 23 Sep 2025 11:55:44 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Paolo Bonzini <pbonzini@...hat.com>
Cc: Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org, 
	Dave Hansen <dave.hansen@...ux.intel.com>, "H. Peter Anvin" <hpa@...or.com>, 
	Ingo Molnar <mingo@...hat.com>, Thomas Gleixner <tglx@...utronix.de>, x86@...nel.org, 
	Borislav Petkov <bp@...en8.de>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] KVM: x86: Fix a semi theoretical bug in kvm_arch_async_page_present_queued

On Tue, Sep 23, 2025, Paolo Bonzini wrote:
> On 8/13/25 21:23, Maxim Levitsky wrote:
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 9018d56b4b0a..3d45a4cd08a4 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -13459,9 +13459,14 @@ void kvm_arch_async_page_present(struct kvm_vcpu *vcpu,
> >   void kvm_arch_async_page_present_queued(struct kvm_vcpu *vcpu)
> >   {
> > -	kvm_make_request(KVM_REQ_APF_READY, vcpu);
> > -	if (!vcpu->arch.apf.pageready_pending)
> > +	/* Pairs with smp_store_release in vcpu_enter_guest. */
> > +	bool in_guest_mode = (smp_load_acquire(&vcpu->mode) == IN_GUEST_MODE);
> > +	bool page_ready_pending = READ_ONCE(vcpu->arch.apf.pageready_pending);
> > +
> > +	if (!in_guest_mode || !page_ready_pending) {
> > +		kvm_make_request(KVM_REQ_APF_READY, vcpu);
> >   		kvm_vcpu_kick(vcpu);
> > +	}
> 
> Unlike Sean, I think the race exists in abstract and is not benign

How is it not benign?  I never said the race doesn't exist, I said that consuming
a stale vcpu->arch.apf.pageready_pending in kvm_arch_async_page_present_queued()
is benign.