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

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Date:   Wed, 21 Jul 2021 10:15:47 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     Marc Zyngier <maz@...nel.org>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Will Deacon <will@...nel.org>,
        Suleiman Souhlal <suleiman@...gle.com>,
        Joel Fernandes <joelaf@...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 (21/07/12 17:24), Marc Zyngier 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.

I suppose you are talking about kvm_vcpu_block(). Well, it checks
kvm_vcpu_check_block() but then it simply schedule() out the vcpu
process, which does look like "the vcpu is preempted". Once we
sched_in() that vcpu process again we mark it as non-preempted,
even though it remains in kvm wfx handler. Why isn't it right?

Another call path is iret:

<iret>
__schedule()
 context_switch()
  prepare_task_switch()
   fire_sched_in_preempt_notifiers()
    kvm_sched_out()
     kvm_arch_vcpu_put()