linux-kernel - Re: [PATCH v3 3/9] KVM: x86: Defer tick-based accounting 'til after IRQ handling

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Message-ID: <20210421100759.GA16580@lothringen>
Date:   Wed, 21 Apr 2021 12:07:59 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        Michael Tokarev <mjt@....msk.ru>,
        Christian Borntraeger <borntraeger@...ibm.com>
Subject: Re: [PATCH v3 3/9] KVM: x86: Defer tick-based accounting 'til after
 IRQ handling

On Thu, Apr 15, 2021 at 03:21:00PM -0700, Sean Christopherson wrote:
> From: Wanpeng Li <wanpengli@...cent.com>
> 
> When using tick-based accounting, defer the call to account guest time
> until after servicing any IRQ(s) that happened in the guest or
> immediately after VM-Exit.  Tick-based accounting of vCPU time relies on
> PF_VCPU being set when the tick IRQ handler runs, and IRQs are blocked
> throughout {svm,vmx}_vcpu_enter_exit().

Out of curiosity, how has guest accounting ever managed to work if we have always
cleared PF_VCPU before re-enabling interrupt?

Thanks.