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

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Message-ID: <3c30e682-a569-9e91-987d-9e2fc66bb625@redhat.com>
Date:   Fri, 10 Dec 2021 14:03:13 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...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 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.

Paolo

  and if the avic errata is present, reduce slightly
> the chances of it happening.