Re: Async page fault delivered while irq are disabled?

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Dec 20, 2019 at 10:34:20AM +0100, Paolo Bonzini wrote:
> On 19/12/19 20:00, Sean Christopherson wrote:
> >> And one last silly question, what about that line in
> >> kvm_arch_can_inject_async_page_present:
> >>
> >> 	if (!(vcpu->arch.apf.msr_val & KVM_ASYNC_PF_ENABLED))
> >> 		return true;
> >>
> >> That looks weird, also it shortcuts the irqs_allowed() check.
> > 
> > I wondered about that code as well :-).  Definitely odd, but it would
> > require the guest to disable async #PF after an async #PF is queued.  Best
> > guess is the idea is that it's the guest's problem if it disables async #PF
> > on the fly.
> > 
> 
> When the guest disables async #PF all outstanding page faults are
> cancelled by kvm_clear_async_pf_completion_queue.  However, in case they
> complete while in cancel_work_sync. you need to inject them even if
> interrupts are disabled.

Hmm, shouldn't the guest wait for the whole pending waitqueue in kvm_async_pf_task_wait()
to be serviced and woken up before actually allowing to disable async #PF ?
Because you can't really afford to inject those #PF while IRQs are disabled,
that's a big rq deadlock risk.