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 14:19:40 +0200 [thread overview]
Message-ID: <20210421121940.GD16580@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.
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.
next prev parent reply other threads:[~2021-04-21 12:19 UTC|newest]
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
2021-04-21 12:19 ` Frederic Weisbecker [this message]
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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