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

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Date:   Fri, 6 Nov 2020 09:32:00 -0700
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        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 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,

Alex