Re: [PATCH] Revert "KVM: x86: Unconditionally enable irqs in guest context"

[Wanpeng Li]

On Thu, 7 Jan 2021 at 17:35, Vitaly Kuznetsov <vkuznets@xxxxxxxxxx> wrote:
>
> Sean Christopherson <seanjc@xxxxxxxxxx> 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
>