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From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Nitesh Narayan Lal <nitesh@redhat.com>,
	linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
	w90p710@gmail.com, pbonzini@redhat.com,
	Thomas Gleixner <tglx@linutronix.de>
Subject: Re: [PATCH] Revert "KVM: x86: Unconditionally enable irqs in guest context"
Date: Thu, 07 Jan 2021 10:33:18 +0100	[thread overview]
Message-ID: <87ble1gkgx.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <X/XvWG18aBWocvvf@google.com>

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).

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.


  reply	other threads:[~2021-01-07  9:35 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 [this message]
2021-01-07  9:41       ` Wanpeng Li
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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