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

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

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@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.
> 

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].

> 
>> 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

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