linux-kernel - Re: [PATCHv2 2/2] kvm: x86: implement KVM PM-notifier

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Date:   Sat, 05 Jun 2021 11:15:51 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Sergey Senozhatsky <senozhatsky@...omium.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Suleiman Souhlal <suleiman@...gle.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCHv2 2/2] kvm: x86: implement KVM PM-notifier

On Sat, 05 Jun 2021 11:03:40 +0100,
Sergey Senozhatsky <senozhatsky@...omium.org> wrote:
> 
> On (21/06/05 18:58), Sergey Senozhatsky wrote:
> [..]
> > > > +static int kvm_arch_suspend_notifier(struct kvm *kvm)
> > > > +{
> > > > +	struct kvm_vcpu *vcpu;
> > > > +	int i, ret;
> > > > +
> > > > +	mutex_lock(&kvm->lock);
> > > > +	kvm_for_each_vcpu(i, vcpu, kvm) {
> > > > +		ret = kvm_set_guest_paused(vcpu);
> > > > +		if (ret) {
> > > > +			pr_err("Failed to pause guest VCPU%d: %d\n",
> > > > +			       vcpu->vcpu_id, ret);
> > > 
> > > Is it really a good idea to fail suspend when a guest doesn't have PV
> > > time enabled? I also wonder how useful the pr_err() is, given that it
> > > contains no information that would help identifying which guest failed
> > > to pause.
> > 
> > No opinion. What shall we do when we fail to suspend the VM?
> > VM's watchdogs will trigger and maybe panic the system after
> > resume.

Panic the guest, maybe. Panic the host, surely not? You cannot decide
what the guest uses or doesn't use anyway, so that's out of your
hands. I don't think this should prevent the host from suspending, in
any case.

> For the time being kvm_set_guest_paused() errors out when
> !vcpu->arch.pv_time_enabled, but this probably can change in the
> future (who knows?).  So shall I check vcpu->arch.pv_time_enabled in
> kvm_arch_suspend_notifier()?

That, or check for the -EINVAL return value.

	M.

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