linux-kernel - Re: [PATCH v2 6/6] kvm: Level IRQ de-assert for KVM

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Date:	Fri, 29 Jun 2012 09:39:51 -0600
From:	Alex Williamson <alex.williamson@...hat.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	mst@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	jan.kiszka@...mens.com
Subject: Re: [PATCH v2 6/6] kvm: Level IRQ de-assert for KVM_IRQFD

On Thu, 2012-06-28 at 15:59 +0300, Avi Kivity wrote:
> On 06/27/2012 08:10 AM, Alex Williamson wrote:
> > This is an alternate level irqfd de-assert mode that's potentially
> > useful for emulated drivers.  It's included here to show how easy it
> > is to implement with the new level irqfd and eoifd support.  It's
> > possible this mode might also prove interesting for device-assignment
> > where we inject via level irqfd, receive an EOI (w/o de-assert), and
> > use the level de-assert irqfd here.
> 
> This use case is racy.  The guest driver will have shut down the
> interrupt before EOI, but with what you describe, it will fire again
> until the eoifd/deassertfd sequence completes.

Hmm, that's a good point.  We'll continue asserting the interrupt in the
indeterminate gap between eoifd and de-assert-irqfd which could fire
enough times for the guest to disable it.  Oh well.  Thanks,

Alex

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