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

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Message-ID: <87a6mf3owt.wl-maz@kernel.org>
Date:   Wed, 21 Jul 2021 12:08:18 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Joel Fernandes <joelaf@...gle.com>, 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 Wed, 21 Jul 2021 11:38:40 +0100,
Sergey Senozhatsky <senozhatsky@...omium.org> wrote:
> 
> On (21/07/21 09:40), Marc Zyngier wrote:
> > > 
> > > 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.
> 
> Can we examine if vcpu->run->exit_reason == WFE/WFI and avoid setting
> preempted state if so?

We never go back to userspace for WFI/WFE, so no reason to populate
the run structure.

Checking for the blocked state is the right thing to do, and we
already have the primitive for this. Just use it.

	M.

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