Re: [PATCH v2 1/2] sched/wait: Add add_wait_queue_priority()

[Paolo Bonzini]

On 04/11/20 10:35, David Woodhouse wrote:
> On Wed, 2020-10-28 at 15:35 +0100, Peter Zijlstra wrote:
> > On Tue, Oct 27, 2020 at 02:39:43PM +0000, David Woodhouse wrote:
> > > From: David Woodhouse <dwmw@xxxxxxxxxxxx>
> > >
> > > This allows an exclusive wait_queue_entry to be added at the head of the
> > > queue, instead of the tail as normal. Thus, it gets to consume events
> > > first without allowing non-exclusive waiters to be woken at all.
> > >
> > > The (first) intended use is for KVM IRQFD, which currently has
> > > inconsistent behaviour depending on whether posted interrupts are
> > > available or not. If they are, KVM will bypass the eventfd completely
> > > and deliver interrupts directly to the appropriate vCPU. If not, events
> > > are delivered through the eventfd and userspace will receive them when
> > > polling on the eventfd.
> > >
> > > By using add_wait_queue_priority(), KVM will be able to consistently
> > > consume events within the kernel without accidentally exposing them
> > > to userspace when they're supposed to be bypassed. This, in turn, means
> > > that userspace doesn't have to jump through hoops to avoid listening
> > > on the erroneously noisy eventfd and injecting duplicate interrupts.
> > >
> > > Signed-off-by: David Woodhouse <dwmw@xxxxxxxxxxxx>
> >
> > Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
>
> Thanks. Paolo, the conclusion was that you were going to take this set
> through the KVM tree, wasn't it?

Yes.

Paolo