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.
--
Vitaly
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
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=87ble1gkgx.fsf@vitty.brq.redhat.com \
--to=vkuznets@redhat.com \
--cc=kvm@vger.kernel.org \
--cc=linux-kernel@vger.kernel.org \
--cc=nitesh@redhat.com \
--cc=pbonzini@redhat.com \
--cc=seanjc@google.com \
--cc=tglx@linutronix.de \
--cc=w90p710@gmail.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).