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From: Wanpeng Li <kernellwp@gmail.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>,
	Nitesh Narayan Lal <nitesh@redhat.com>,
	LKML <linux-kernel@vger.kernel.org>, kvm <kvm@vger.kernel.org>,
	w90p710@gmail.com, Paolo Bonzini <pbonzini@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] Revert "KVM: x86: Unconditionally enable irqs in guest context"
Date: Thu, 7 Jan 2021 17:41:03 +0800	[thread overview]
Message-ID: <CANRm+CzXOiWV1dUQiN69TZijifBqiNoJ-b6z58yoGw51Pu1+6g@mail.gmail.com> (raw)
In-Reply-To: <87ble1gkgx.fsf@vitty.brq.redhat.com>

On Thu, 7 Jan 2021 at 17:35, Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
>
> Sean Christopherson <seanjc@google.com> writes:
>
> > On Wed, Jan 06, 2021, Vitaly Kuznetsov wrote:
> >>
> >> Looking back, I don't quite understand why we wanted to account ticks
> >> between vmexit and exiting guest context as 'guest' in the first place;
> >> to my understanging 'guest time' is time spent within VMX non-root
> >> operation, the rest is KVM overhead (system).
> >
> > With tick-based accounting, if the tick IRQ is received after PF_VCPU is cleared
> > then that tick will be accounted to the host/system.  The motivation for opening
> > an IRQ window after VM-Exit is to handle the case where the guest is constantly
> > exiting for a different reason _just_ before the tick arrives, e.g. if the guest
> > has its tick configured such that the guest and host ticks get synchronized
> > in a bad way.
> >
> > This is a non-issue when using CONFIG_VIRT_CPU_ACCOUNTING_GEN=y, at least with a
> > stable TSC, as the accounting happens during guest_exit_irqoff() itself.
> > Accounting might be less-than-stellar if TSC is unstable, but I don't think it
> > would be as binary of a failure as tick-based accounting.
> >
>
> Oh, yea, I vaguely remember we had to deal with a very similar problem
> but for userspace/kernel accounting. It was possible to observe e.g. a
> userspace task going 100% kernel while in reality it was just perfectly
> synchronized with the tick and doing a syscall just before it arrives
> (or something like that, I may be misremembering the details).

Yes. :)  commit 2a42eb9594a1 ("sched/cputime: Accumulate vtime on top
of nsec clocksource")

> So depending on the frequency, it is probably possible to e.g observe
> '100% host' with tick based accounting, the guest just has to
> synchronize exiting to KVM in a way that the tick will always arrive
> past guest_exit_irqoff().
>
> It seems to me this is a fundamental problem in case the frequency of
> guest exits can match the frequency of the time accounting tick.
>
> --
> Vitaly
>

  reply	other threads:[~2021-01-07  9:42 UTC|newest]

Thread overview: 13+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-01-05 19:28 [PATCH] Revert "KVM: x86: Unconditionally enable irqs in guest context" Nitesh Narayan Lal
2021-01-06  0:42 ` Wanpeng Li
2021-01-06  0:47 ` Sean Christopherson
2021-01-06  1:35   ` Nitesh Narayan Lal
2021-01-15  3:20   ` Wanpeng Li
2021-01-19  1:27     ` Wanpeng Li
2021-01-06 10:09 ` Vitaly Kuznetsov
2021-01-06 17:11   ` Sean Christopherson
2021-01-07  9:33     ` Vitaly Kuznetsov
2021-01-07  9:41       ` Wanpeng Li [this message]
2021-01-12 21:43       ` Nitesh Narayan Lal
2021-01-12 22:04         ` Sean Christopherson
2021-01-07 10:55     ` Xinlong Lin

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