From: Vitaly Kuznetsov <firstname.lastname@example.org>
To: Sean Christopherson <email@example.com>
Cc: Nitesh Narayan Lal <firstname.lastname@example.org>,
Thomas Gleixner <email@example.com>
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: <firstname.lastname@example.org> (raw)
Sean Christopherson <email@example.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
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.
next prev parent 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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