Re: [patch uq/master 1/2] virtio-pci: wake up iothread on VIRTIO_PCI_QUEUE_NOTIFY

[Anthony Liguori <anthony@codemonkey.ws>]

On 02/22/2010 09:16 AM, Marcelo Tosatti wrote:
>>> Are you concerned about spurious wakeups?
>>>        
>> Yes.  Also, qemu_notify_event() is an undirected notification (wakes
>> up all iothreads, and all devices), whereas ->handle_output() is
>> directed (wakes up exactly what is needed).
>>
>> What's the underlying problem?  A new input buffer has become
>> available, and we need to re-poll the incoming file descriptor?  If
>> so, that's best done from ->handle_output() (either by waking the
>> iothread or calling read() itself and perhaps receiving -EAGAIN).
>>      
> Yes. Sure, perhaps calling read() itself is appropriate, and i see
> your point that>handle_output contains more context for a smarter
> decision.
>
> But one can argue thats an improvement on top of a dumb wakeup.
>    

Spurious calls to qemu_notify_event() also make it difficult to tell 
when it's actually necessary to call qemu_notify_event() vs. when it's 
just something that doesn't hurt.

Regards,

Anthony Liguori

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