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From: Frederic Weisbecker <frederic@kernel.org>
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Jim Mattson <jmattson@google.com>, Joerg Roedel <joro@8bytes.org>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Michael Tokarev <mjt@tls.msk.ru>,
	Christian Borntraeger <borntraeger@de.ibm.com>
Subject: Re: [PATCH v3 3/9] KVM: x86: Defer tick-based accounting 'til after IRQ handling
Date: Wed, 21 Apr 2021 12:11:58 +0200
Message-ID: <20210421101158.GB16580@lothringen> (raw)
In-Reply-To: <YH9jKpeviZtMKxt8@google.com>

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.

Ok let's keep it as is then. It reads funny but can we perhaps hope that
other archs have the same issue and in the future we'll need to have this split
generalized outside x86 with:

	* guest_exit_vtime()
	* guest_exit_tick()

  reply index

Thread overview: 25+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-15 22:20 [PATCH v3 0/9] KVM: Fix tick-based accounting for x86 guests Sean Christopherson
2021-04-15 22:20 ` [PATCH v3 1/9] context_tracking: Move guest exit context tracking to separate helpers Sean Christopherson
2021-04-20 18:48   ` Christian Borntraeger
2021-04-21 10:57   ` Frederic Weisbecker
2021-04-15 22:20 ` [PATCH v3 2/9] context_tracking: Move guest exit vtime accounting " Sean Christopherson
2021-04-20 18:48   ` Christian Borntraeger
2021-04-15 22:21 ` [PATCH v3 3/9] KVM: x86: Defer tick-based accounting 'til after IRQ handling Sean Christopherson
2021-04-20 23:14   ` Frederic Weisbecker
2021-04-20 23:26     ` Sean Christopherson
2021-04-21 10:11       ` Frederic Weisbecker [this message]
2021-04-21 12:19       ` Frederic Weisbecker
2021-04-28 22:38         ` Sean Christopherson
2021-04-21 10:07   ` Frederic Weisbecker
2021-04-15 22:21 ` [PATCH v3 4/9] sched/vtime: Move vtime accounting external declarations above inlines Sean Christopherson
2021-04-21  7:02   ` Christian Borntraeger
2021-04-15 22:21 ` [PATCH v3 5/9] sched/vtime: Move guest enter/exit vtime accounting to vtime.h Sean Christopherson
2021-04-15 22:21 ` [PATCH v3 6/9] context_tracking: Consolidate guest enter/exit wrappers Sean Christopherson
2021-04-15 22:21 ` [PATCH v3 7/9] context_tracking: KVM: Move guest enter/exit wrappers to KVM's domain Sean Christopherson
2021-04-21  7:10   ` Christian Borntraeger
2021-04-15 22:21 ` [PATCH v3 8/9] KVM: x86: Consolidate guest enter/exit logic to common helpers Sean Christopherson
2021-04-15 22:21 ` [PATCH v3 9/9] KVM: Move instrumentation-safe annotations for enter/exit to x86 code Sean Christopherson
2021-04-21  8:09   ` Christian Borntraeger
2021-04-22 14:38     ` Sven Schnelle
2021-04-23  9:32       ` Vasily Gorbik
2021-04-20 23:33 ` [PATCH v3 0/9] KVM: Fix tick-based accounting for x86 guests Frederic Weisbecker

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