Re: [Qemu-devel] [PATCH 00/10] block: Coroutine support

[Anthony Liguori <anthony@codemonkey.ws>]

On 08/02/2011 09:50 AM, Kevin Wolf wrote:
> Am 02.08.2011 16:23, schrieb Avi Kivity:
>> On 07/26/2011 02:48 PM, Kevin Wolf wrote:
>>> Depends on Stefan's latest coroutine patches. This series makes qcow and qcow2
>>> take advantage of the new coroutine infrastructure. Both formats used
>>> synchronous operations for accessing their metadata and blocked the guest CPU
>>> during that time. With coroutines, the I/O will happen asynchronously in the
>>> background and the CPU won't be blocked any more.
>>>
>>
>> Do you plan to convert qcow2 to a fully synchronous design?
>>
>> IMO that will make it more maintainable.  Cancellation will need some
>> thought, though.
>
> After this patch series, all interesting paths are free of callbacks (I
> assume this is what you mean by synchronous?). The only thing I can see
> that is left is qcow2_aio_flush. What is required are some cleanups that
> eliminate things that still look like AIO code, and yes, that's
> something that I want to have.
>
> Frediano has posted some patches which I haven't fully reviewed yet, but
> the qcow1 RFC he posted was definitely a step in the right direction.
>
> Regarding cancellation, I don't know any driver that really does what
> it's supposed to do. There are basically two ways of implementing it in
> current code: Either by completing the request instead of cancelling, or
> it's broken. I'd suggest that we implement waiting for completion as a
> generic function in the block layer and be done with it (actually this
> is what happens with bdrv_aio_co_cancel_em, it just could be a bit finer
> grained).

If you introduce queuing at the generic layer, than removing from queue 
or waiting for completion is entirely acceptable semantics IMHO.

Regards,

Anthony Liguori

>
> Kevin
>