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

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Date:	Tue, 19 Jan 2010 16:04:25 +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" <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 03:03:34PM +0100, Jan Kiszka wrote:
> Michael S. Tsirkin wrote:
> > 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.
> > 
> 
> OK...
> 
> >> 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?
> 
> Now that you mention it: not yet properly. So far we pass the file
> struct as pseudo eventfd_ctx around on < 2.6.31. But now that I peek
> into the struct in kvm_eventfd_ctx_remove_wait_queue, this should should
> crash. Guess I need to look up that module the same way as I acquire
> __wake_up_locked[_key].

This won't work that well: eventfd in upstream
sends us POLLHUP so we can close the structure,
in old kernels it doesn't so kernel will crash
when we try to reference the structure later.


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