KVM exit to userspace on WFI

[Jan Henrik Weinstock]

Hi Marc,

the basic idea behind this is to have a (single-threaded) execution loop,
something like this:

vcpu-thread:    vcpu-run | process-io-devices | vcpu-run | process-io...
                         ^
                  WFX or timeout

We switch to simulating IO devices whenever the vcpu is idle (wfi) or exceeds
a certain budget of instructions (counted via pmu). Our fallback currently is
to kick the vcpu out of its execution using a signal (via a timeout/alarm). But
of course, if the cpu is stuck at a wfi, we are wasting a lot of time.

I understand that the proposed behavior is not desirable for most use cases,
which is why I suggest locking it behind a flag, e.g.
KVM_ARCH_FLAG_WFX_EXIT_TO_USER.


Am Mi., 25. Okt. 2023 um 14:42 Uhr schrieb Marc Zyngier <maz at kernel.org>:
>
> On Wed, 25 Oct 2023 13:12:14 +0100,
> Jan Henrik Weinstock <jan at mwa.re> wrote:
> >
> > Hi Marc,
> >
> > Thanks for your feedback. I understand that request_interrupt_window
> > is not to be used. I assume a setting a flag is a better way,
> > something similar to KVM_ARCH_FLAG_RETURN_NISV_IO_ABORT_TO_USER, e.g.
> > KVM_ARCH_FLAG_WFX_EXIT_TO_USER.
> >
> > I will also check that WFx traps are always enabled while this mode is
> > active to make sure userspace does not get blocked/scheduled out.
>
> Why would that be an acceptable behaviour?
>
> > The reason for this is that we cannot have the thread that executes
> > KVM_RUN to be blocked or scheduled out whenever it hits a WFI.
>
> Why? If that's not acceptable, how do you even cope with the basic
> preemption?
>
> > Nop-WFIs are not a problem, since the PE will just continue executing
> > instructions, which is fine. We are currently using a timeout signal
> > that kicks KVM_RUN back into userspace, but we are seeing a lot of
> > time wasted because our KVM thread hangs in WFI/WFEs. It would be
> > better if we could just return from KVM_RUN immediately if the thread
> > would otherwise be blocked.
>
> On the face of it, this makes little sense:
>
> - While in userspace, no interrupt source that normally delivered
>   without any userpsace intervention will be blocked (timers,
>   VLPIs...). I cannot how this can be a good idea.
>
> - Trapping WFE is an important scheduling hint, and returning to
>   userspace defeats it. Contended spinlocks, for example, will be even
>   slower to acquire.
>
> I'm sure you have a particular use case for such a degraded behaviour,
> but since you are not describing it, I'm not at all inclined to
> actively break KVM's performance and scalability.
>
> Thanks,
>
>         M.
>
> --
> Without deviation from the norm, progress is not possible.



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