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

Michael S. Tsirkin
2010-01-19T13:51:32Z

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@redhat.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.

> --
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