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

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Date:   Wed, 21 Apr 2021 14:19:40 +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 Tue, Apr 20, 2021 at 11:26:34PM +0000, Sean Christopherson wrote:
> 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.

Ah I see, before 87fa7f3e98a131 the vtime was accounted after interrupts get
processed. So it used to work until then. I see that ARM64 waits for IRQs to
be enabled too.

PPC/book3s_hv, MIPS, s390 do it before IRQs get re-enabled (weird, how does that
work?)

And PPC/book3s_pr calls guest_exit() so I guess it has interrupts enabled.

The point is: does it matter to call vtime_account_guest_exit() before or
after interrupts? If it doesn't matter, we can simply call
vtime_account_guest_exit() once and for all once IRQs are re-enabled.

If it does matter because we don't want to account the host IRQs firing at the
end of vcpu exit, then probably we should standardize that behaviour and have
guest_exit_vtime() called before interrupts get enabled and guest_exit_tick()
called after interrupts get enabled. It's probably then beyond the scope of this
patchset but I would like to poke your opinion on that.

Thanks.