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

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Date:   Tue, 20 Apr 2021 23:26:34 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Frederic Weisbecker <frederic@...nel.org>
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 Wed, Apr 21, 2021, Frederic Weisbecker wrote:
> On Thu, Apr 15, 2021 at 03:21:00PM -0700, Sean Christopherson wrote:
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 16fb39503296..e4d475df1d4a 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -9230,6 +9230,14 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >  	local_irq_disable();
> >  	kvm_after_interrupt(vcpu);
> >  
> > +	/*
> > +	 * When using tick-based accounting, wait until after servicing IRQs to
> > +	 * account guest time so that any ticks that occurred while running the
> > +	 * guest are properly accounted to the guest.
> > +	 */
> > +	if (!vtime_accounting_enabled_this_cpu())
> > +		vtime_account_guest_exit();
> 
> Can we rather have instead:
> 
> static inline void tick_account_guest_exit(void)
> {
> 	if (!vtime_accounting_enabled_this_cpu())
> 		current->flags &= ~PF_VCPU;
> }
> 
> It duplicates a bit of code but I think this will read less confusing.

Either way works for me.  I used vtime_account_guest_exit() to try to keep as
many details as possible inside vtime, e.g. in case the implemenation is tweaked
in the future.  But I agree that pretending KVM isn't already deeply intertwined
with the details is a lie.