linux-kernel - Re: [PATCHv2 4/4] arm64: add host pv-vcpu-state support

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Date:   Wed, 21 Jul 2021 09:40:46 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Joel Fernandes <joelaf@...gle.com>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Will Deacon <will@...nel.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org
Subject: Re: [PATCHv2 4/4] arm64: add host pv-vcpu-state support

On Tue, 20 Jul 2021 19:44:53 +0100,
Joel Fernandes <joelaf@...gle.com> wrote:
> 
> On Mon, Jul 12, 2021 at 12:24 PM Marc Zyngier <maz@...nel.org> wrote:
> >
> [...]
> > >  void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
> > >  {
> > > +     kvm_update_vcpu_preempted(vcpu, true);
> >
> > This doesn't look right. With this, you are now telling the guest that
> > a vcpu that is blocked on WFI is preempted. This really isn't the
> > case, as it has voluntarily entered a low-power mode while waiting for
> > an interrupt. Indeed, the vcpu isn't running. A physical CPU wouldn't
> > be running either.
> 
> Can that be cured by just checking vcpu->preempted before calling
> kvm_update_vcpu_preempted() ?

It isn't obvious to me that this is the right thing to do.
vcpu->preempted is always updated on sched-out from the preempt
notifier if the vcpu was on the run-queue, so my guess is that it will
always be set when switching to another task.

What you probably want is to check whether the vcpu is blocked by
introspecting the wait-queue with:

	scuwait_active(kvm_arch_vcpu_get_wait(vcpu)

which will tell you whether you are blocking or not. We are already
using a similar construct for arming a background timer in this case.

	M.

-- 
Without deviation from the norm, progress is not possible.