linux-kernel - Re: [PATCH 5/6] KVM: x86: never clear irr_pending in kvm_apic_update

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Date:   Fri, 10 Dec 2021 15:10:34 +0200
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Cc:     "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>, Wanpeng Li <wanpengli@...cent.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Borislav Petkov <bp@...en8.de>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jim Mattson <jmattson@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH 5/6] KVM: x86: never clear irr_pending in
 kvm_apic_update_apicv

On Fri, 2021-12-10 at 14:03 +0100, Paolo Bonzini wrote:
> On 12/10/21 13:47, Maxim Levitsky wrote:
> > If we scan vIRR here and see no bits, and*then*  disable AVIC,
> > there is a window where the they could legit be turned on without any cpu errata,
> > and we will not have irr_pending == true, and thus the following
> > KVM_REQ_EVENT will make no difference.
> 
> Right.
> 
> > Not touching irr_pending and letting just the KVM_REQ_EVENT do the work
> > will work too,
> 
> Yeah, I think that's preferrable.  irr_pending == true is a conservative 
> setting that works; irr_pending will be evaluated again on the first 
> call to apic_clear_irr and that's enough.
> 
> With that justification, you don't need to reorder the call to 
> kvm_apic_update_apicv to be after kvm_x86_refresh_apicv_exec_ctrl.

Yes exactly! but no need to scan IRR here since irr_pending is already
true at that point anyway - it is always true while avic is enabled.


Best regards,
	Maxim Levitsky
> 
> Paolo
> 
>   and if the avic errata is present, reduce slightly
> > the chances of it happening.