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

David Woodhouse
2020-11-04T09:35:45Z

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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@amazon.co.uk>
> >
> > 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@amazon.co.uk>
>
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>

Thanks. Paolo, the conclusion was that you were going to take this set
through the KVM tree, wasn't it?

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