Re: [PATCH] KVM: arm64: vgic-v4: Consistently request doorbell irq for blocking vCPU

[Marc Zyngier <maz@kernel.org>]

On Tue, 11 Jul 2023 08:26:54 +0100,
Oliver Upton <oliver.upton@linux.dev> wrote:
> 
> On Tue, Jul 11, 2023 at 08:23:25AM +0100, Marc Zyngier wrote:
> > On Mon, 10 Jul 2023 18:55:53 +0100,
> > Oliver Upton <oliver.upton@linux.dev> wrote:
> > > 
> > > Xiang reports that VMs occasionally fail to boot on GICv4.1 systems when
> > > running a preemptible kernel, as it is possible that a vCPU is blocked
> > > without requesting a doorbell interrupt.
> > > 
> > > The issue is that any preemption that occurs between vgic_v4_put() and
> > > schedule() on the block path will mark the vPE as nonresident and *not*
> > > request a doorbell irq.
> > 
> > It'd be worth spelling out. You need to go via *three* schedule()
> > calls: one to be preempted (with DB set), one to be made resident
> > again, and then the final one in kvm_vcpu_halt(), clearing the DB on
> > vcpu_put() due to the bug.
> 
> Yeah, a bit lazy in the wording. What I had meant to imply was
> preemption happening after the doorbell is set up and before the thread
> has an opportunity to explicitly schedule out. Perhaps I should just say
> that.

Yup. And it is the transition via a new 'resident' state that blows
it. No need to repost for that, just amend it locally.

Thanks,

	M.

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