Re: [Qemu-devel] [PATCH] event_notifier: move to top-level directory

[Stefan Hajnoczi]

On Tue, Sep 27, 2011 at 10:23 PM, Anthony Liguori <address@hidden> wrote:
> On 09/27/2011 11:39 AM, Paolo Bonzini wrote:
>>
>> On 09/27/2011 06:05 PM, Anthony Liguori wrote:
>>>
>>> Actually, for posix-aio, we can just switch to using g_idle_add().
>>> g_idle_add() uses g_source_attach which is thread safe. g_idle_add()
>>> gives you a thread safe mechanism to defer a piece of work to the
>>> main loop which is really what we want here.
>>
>> For that, a bottom half would also do (apart that I am not sure it is
>> async-safe with TCG). In fact, that would make sense since all of
>> posix_aio_process_queue could become a bottom half.
>
> Bottom halves are signal safe, not thread safe.
>
> To make bottom halves thread safe, you would (in the very least) have to add
> some barriers when reading/writing the scheduling flag.  I think it's much
> better to just use GIdle sources though.
>
>>> This can actually be made to work with sync I/O emulation too by
>>> having another GMainLoop in the sync I/O loop although I thought I
>>> recalled a patch series to remove that stuff.
>>
>> ... which stuff? :)
>
> The sync I/O emulation.  Since sync I/O is done in block drivers, they can
> just use coroutine I/O instead of sync I/O.

Yes, I think we should covert sync I/O code to use coroutines, which
is quite natural.  The users of sync I/O today are:
1. Hardware emulation - lesser-used or not performance-critical code
paths still use bdrv_read/write() in places, e.g. sd, nand, fdc, ide
pio.
2. Block drivers.  Some image formats are synchronous, but converting
to coroutines is pretty easy.
3. qemu-tools including qemu-img and qemu-io.

With Paolo's work to make the event loop available in qemu-tools it
should be possible to convert and eliminate synchronous I/O
interfaces.

Stefan