linux-kernel - Re: [PATCH 1/2] kvm: fix spurious interrupt with irqfd

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Date:	Tue, 19 Jan 2010 15:48:27 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	Davide Libenzi <davidel@...ilserver.org>,
	Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] kvm: fix spurious interrupt with irqfd

On Tue, Jan 19, 2010 at 02:25:12PM +0100, Jan Kiszka wrote:
> Michael S. Tsirkin wrote:
> > kvm didn't clear irqfd counter on deassign, as a result we could get a
> > spurious interrupt when irqfd is assigned back. this leads to poor
> > performance and, in theory, guest crash.
> > 
> > Signed-off-by: Michael S. Tsirkin <mst@...hat.com>
> > ---
> >  virt/kvm/eventfd.c |    3 ++-
> >  1 files changed, 2 insertions(+), 1 deletions(-)
> > 
> > diff --git a/virt/kvm/eventfd.c b/virt/kvm/eventfd.c
> > index 62e4cd9..a9d3fc6 100644
> > --- a/virt/kvm/eventfd.c
> > +++ b/virt/kvm/eventfd.c
> > @@ -72,12 +72,13 @@ static void
> >  irqfd_shutdown(struct work_struct *work)
> >  {
> >  	struct _irqfd *irqfd = container_of(work, struct _irqfd, shutdown);
> > +	u64 cnt;
> >  
> >  	/*
> >  	 * Synchronize with the wait-queue and unhook ourselves to prevent
> >  	 * further events.
> >  	 */
> > -	remove_wait_queue(irqfd->wqh, &irqfd->wait);
> > +	eventfd_ctx_remove_wait_queue(irqfd->eventfd, &irqfd->wait, &cnt);
> >  
> >  	/*
> >  	 * We know no new events will be scheduled at this point, so block
> 
> For kvm-kmod, I'm fighting with compat support for
> eventfd_ctx_remove_wait_queue. I basically have a solution for kernels
> with CONFIG_KPROBES enabled (I need to look up unexported
> __wake_up_locked[_key]), but there will also be target kernels that do
> not have this. So there are three options for that case:
> 
>  - Warn the user and fall back to the old racy approach
>  - (Somehow) disable KVM subsystems that use eventfd
>  - Refuse to start KVM
> As far as I understood, irqfd is interesting for device assignment and
> now also for vhost, right?

At the moment, only vhost.

> What about ioeventfd?

Same thing.

> I just wonder how broad
> the impact of a broken or non-existent eventfd subsystem for kvm-kmod
> is. Any thoughts welcome.

How do you handle kernels that don't export eventfd_ctx_fileget?

> Jan
> 
> PS: If anyone forgot why Avi handed over this job, you should now
> remember why. :)

Heh, I did the same kind of thing for infiniband for
several years. It's hard to forget.

> -- 
> Siemens AG, Corporate Technology, CT T DE IT 1
> Corporate Competence Center Embedded Linux
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