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

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Message-ID: <F67D16D9-A5F0-4B40-8E1F-E713174D997B@infradead.org>
Date:   Fri, 06 Nov 2020 17:18:08 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Alex Williamson <alex.williamson@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>
CC:     Peter Zijlstra <peterz@...radead.org>,
        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>,
        kvm@...r.kernel.org
Subject: Re: [PATCH v2 1/2] sched/wait: Add add_wait_queue_priority()



On 6 November 2020 16:32:00 GMT, Alex Williamson <alex.williamson@...hat.com> wrote:
>On Fri, 6 Nov 2020 11:17:21 +0100
>Paolo Bonzini <pbonzini@...hat.com> wrote:
>
>> 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@...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?
>> >   
>> 
>> Queued, except for patch 2/3 in the eventfd series which Alex hasn't 
>> reviewed/acked yet.
>
>There was no vfio patch here, nor mention why it got dropped in v2
>afaict.  Thanks,

That was a different (but related) series. The VFIO one is https://patchwork.kernel.org/project/kvm/patch/20201027135523.646811-3-dwmw2@infradead.org/

Thanks.

-- 
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