linux-kernel - Re: [PATCH v2] KVM: x86: check PIR even for vCPUs with disabled APICv

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Date:   Thu, 18 Nov 2021 16:18:26 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        stable@...r.kernel.org
Subject: Re: [PATCH v2] KVM: x86: check PIR even for vCPUs with disabled APICv

On Thu, Nov 18, 2021, Paolo Bonzini wrote:
> The IRTE for an assigned device can trigger a POSTED_INTR_VECTOR even
> if APICv is disabled on the vCPU that receives it.  In that case, the
> interrupt will just cause a vmexit and leave the ON bit set together
> with the PIR bit corresponding to the interrupt.
> 
> Right now, the interrupt would not be delivered until APICv is re-enabled.
> However, fixing this is just a matter of always doing the PIR->IRR
> synchronization, even if the vCPU has temporarily disabled APICv.
> 
> This is not a problem for performance, or if anything it is an
> improvement.  First, in the common case where vcpu->arch.apicv_active is
> true, one fewer check has to be performed.  Second, static_call_cond will
> elide the function call if APICv is not present or disabled.  Finally,
> in the case for AMD hardware we can remove the sync_pir_to_irr callback:
> it is only needed for apic_has_interrupt_for_ppr, and that function
> already has a fallback for !APICv.
> 
> Cc: stable@...r.kernel.org
> Co-developed-by: Sean Christopherson <seanjc@...gle.com>

For my bits:

Signed-off-by: Sean Christopherson <seanjc@...gle.com>