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Re: [Qemu-devel] [RFC] block: Removed coroutine ownership assumption

From: Kevin Wolf
Subject: Re: [Qemu-devel] [RFC] block: Removed coroutine ownership assumption
Date: Mon, 02 Jul 2012 11:04:07 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:13.0) Gecko/20120605 Thunderbird/13.0

Am 02.07.2012 10:57, schrieb Peter Crosthwaite:
> On Mon, Jul 2, 2012 at 6:50 PM, Stefan Hajnoczi <address@hidden> wrote:
>> On Fri, Jun 29, 2012 at 12:51 PM, Peter Crosthwaite
>> <address@hidden> wrote:
>>> BTW Yielding is one thing, but the elephant in the room here is
>>> resumption of the coroutine. When AIO yields my coroutine I i need to
>>> talk to AIO to get it unyielded (Stefans propsoed edit to my code).
>>> What happens when tommorow something in QOM, or a device model or  is
>>> implemented with coros too? how do I know who yielded my routines and
>>> what API to call re-enter it?
>> Going back to what Kevin said, the qemu_aio_wait() isn't block layer
>> specific and there will never be a requirement to call any other magic
>> functions.
>> QEMU is event-driven and you must pump events.  If you call into
>> another subsystem, be prepared to pump events so that I/O can make
>> progress.  It's an assumption that is so central to QEMU architecture
>> that I don't see it as a problem.
>> Once the main loop is running then the event loop is taken care of for
>> you.  But during startup you're in a different environment and need to
>> pump events yourself.
>> If we drop bdrv_read()/bdrv_write() this won't change.  You'll have to
>> call bdrv_co_readv()/bdrv_co_writev() and pump events.
> Just tracing bdrv_aio_read(), It bypasses the fastpath logic:
> . So a conversion of pflash to bdrv_aio_readv is a possible solution here.
> bdrv_aio_read -> bdrv_co_aio_rw_vector :
> static BlockDriverAIOCB *bdrv_co_aio_rw_vector (..) {
>     Coroutine *co;
>     BlockDriverAIOCBCoroutine *acb;
>     ...
>     co = qemu_coroutine_create(bdrv_co_do_rw);
>     qemu_coroutine_enter(co, acb);
>     return &acb->common;
> }
> No conditional on the qemu_coroutine_create. So it will always create
> a new coroutine for its work which will solve my problem. All I need
> to do is pump events once at the end of machine model creation. Any my
> coroutines will never yield or get queued by block/AIO. Sound like a
> solution?

If you don't need the read data in your initialisation code, then yes,
that would work. bdrv_aio_* will always create a new coroutine. I just
assumed that you wanted to use the data right away, and then using the
AIO functions wouldn't have made much sense.


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