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

[Jan Kiszka <jan.kiszka@siemens.com>]

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

Jan

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

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