linux-kernel - Re: [PATCH v2 1/2] sched/wait: Add add_wait_queue

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Message-ID: <ef4660dba8135ca5a1dc7e854babcf65d8cef46f.camel@infradead.org>
Date:   Wed, 04 Nov 2020 09:35:37 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org
Subject: Re: [PATCH v2 1/2] sched/wait: Add add_wait_queue_priority()

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@...zon.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@...zon.co.uk>
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@...radead.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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