linux-kernel - Re: KVM exit to userspace on WFI

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Date:   Wed, 25 Oct 2023 13:42:30 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Jan Henrik Weinstock <jan@....re>
Cc:     oliver.upton@...ux.dev, james.morse@....com,
        suzuki.poulose@....com, yuzenghui@...wei.com,
        catalin.marinas@....com, will@...nel.org,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        Lukas Jünger <lukas@....re>
Subject: Re: KVM exit to userspace on WFI

On Wed, 25 Oct 2023 13:12:14 +0100,
Jan Henrik Weinstock <jan@....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.