Re: [GIT PULL] AlacrityVM guest drivers for 2.6.33

Avi Kivity
2009-12-22T19:25:20Z

On 12/22/2009 09:15 PM, Gregory Haskins wrote:
> On 12/22/09 1:53 PM, Avi Kivity wrote:
>
>> I asked why the irqfd/ioeventfd mechanisms are insufficient, and you did not reply.
>>
>>
> BTW: the ioeventfd issue just fell through the cracks, so sorry about
> that. Note that I have no specific issue with irqfd ever since the
> lockless IRQ injection code was added.
>
> ioeventfd turned out to be suboptimal for me in the fast path for two
> reasons:
>
> 1) the underlying eventfd is called in atomic context. I had posted
> patches to Davide to address that limitation, but I believe he rejected
> them on the grounds that they are only relevant to KVM.
>

If you're not doing something pretty minor, you're better of waking up a
thread (perhaps _sync if you want to keep on the same cpu). With the
new user return notifier thingie, that's pretty cheap.

> 2) it cannot retain the data field passed in the PIO. I wanted to have
> one vector that could tell me what value was written, and this cannot be
> expressed in ioeventfd.
>
>

It would be easier to add data logging support to ioeventfd, if it was
needed that badly.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.