Re: [Qemu-devel] [PATCH v2 11/11] block: Only poll block layer fds in bdrv_aio_poll

[Fam Zheng]

On Wed, 07/29 14:03, Paolo Bonzini wrote:
> 
> 
> On 29/07/2015 13:53, Fam Zheng wrote:
> >> > Yes, though I think you'd end up reverting patches 10 and 11 in the end.
> > We will add outer disable/enable pairs to prevent another threads's aio_poll
> > from sneaking in between bdrv_aio_poll calls, but we needn't obsolete
> > bdrv_aio_poll() because of that - it can be useful by itself. For example
> > bdrv_aio_cancel shouldn't look at ioeventfd, otherwise it could spin for too
> > long on high load.  Does that make sense?
> 
> Did you mean bdrv_drain() (when it is not already surrounded by
> disable/enable pairs in the caller)?  But yes, that makes sense.
> 
> I'm not sure that it makes sense to disable/enable in places such as
> bdrv_pread.  The caller's block, if any, should suffice.  In this sense
> you'd end up reverting large parts of patch 10.
> 
> Then you would have to see how many calls to bdrv_aio_poll are still
> there, and how many can be converted with no semantic change to aio_poll
> (e.g. there's no difference in qemu-img.c), and you'd end up reverting
> patches 9 and 11 too.  But we can look at that later.

Another advantage for bdrv_aio_poll() is, in main loop we will not need
a separate AioContext in changes like:

http://patchwork.ozlabs.org/patch/514968/

Because nested aio_poll will automatically be limited to only process block
layer events. My idea is to eventually let main loop use aio_poll, which means
we would also move chardev onto it. It would be neat to put all fds of the main
thread into a single AioContext.

Fam