Il 09/10/2012 17:37, Anthony Liguori ha scritto:
>>> >> In the very short term, I can imagine an aio fastpath that was only
>>> >> implemented in terms of the device API. We could have a slow path that
>>> >> acquired the BQL.
>> >
>> > Not sure I follow.
>
> As long as the ioeventfd thread can acquire qemu_mutex in order to call
> bdrv_* functions. The new device-only API could do this under the
> covers for everything but the linux-aio fast path initially.
Ok, so it's about the locking. I'm not even sure we need locking if we
have cooperative multitasking. For example if bdrv_aio_readv/writev
is called from a VCPU thread, it can just schedule a bottom half for
itself in the appropriate AioContext. Similarly for block jobs.
The only part where I'm not sure how it would work is bdrv_read/write,
because of the strange "qemu_aio_wait() calls select with a lock taken".
Maybe we can just forbid synchronous I/O if you set a non-default
AioContext.
This would be entirely hidden in the block layer. For example the
following does it for bdrv_aio_readv/writev:
diff --git a/block.c b/block.c
index e95f613..7165e82 100644
--- a/block.c
+++ b/block.c
@@ -3712,15 +3712,6 @@ static AIOPool bdrv_em_co_aio_pool = {
.cancel = bdrv_aio_co_cancel_em,
};
-static void bdrv_co_em_bh(void *opaque)
-{
- BlockDriverAIOCBCoroutine *acb = opaque;
-
- acb->common.cb(acb->common.opaque, acb->req.error);
- qemu_bh_delete(acb->bh);
- qemu_aio_release(acb);
-}
-
/* Invoke bdrv_co_do_readv/bdrv_co_do_writev */
static void coroutine_fn bdrv_co_do_rw(void *opaque)
{
@@ -3735,8 +3726,17 @@ static void coroutine_fn bdrv_co_do_rw(void *opaque)
acb->req.nb_sectors, acb->req.qiov, 0);
}
- acb->bh = qemu_bh_new(bdrv_co_em_bh, acb);
- qemu_bh_schedule(acb->bh);
+ acb->common.cb(acb->common.opaque, acb->req.error);
+ qemu_aio_release(acb);
+}
+
+static void bdrv_co_em_bh(void *opaque)
+{
+ BlockDriverAIOCBCoroutine *acb = opaque;
+
+ qemu_bh_delete(acb->bh);
+ co = qemu_coroutine_create(bdrv_co_do_rw);
+ qemu_coroutine_enter(co, acb);
}
static BlockDriverAIOCB *bdrv_co_aio_rw_vector(BlockDriverState *bs,
@@ -3756,8 +3756,8 @@ static BlockDriverAIOCB
*bdrv_co_aio_rw_vector(BlockDriverState *bs,
acb->req.qiov = qiov;
acb->is_write = is_write;
- co = qemu_coroutine_create(bdrv_co_do_rw);
- qemu_coroutine_enter(co, acb);
+ acb->bh = qemu_bh_new(bdrv_co_em_bh, acb);
+ qemu_bh_schedule(acb->bh);
return &acb->common;
}
Then we can add a bdrv_aio_readv/writev_unlocked API to the protocols, which
would run outside the bottom half and provide the desired fast path.
Paolo
> That means that we can convert block devices to use the device-only API
> across the board (provided we make BQL recursive).
>
> It also means we get at least some of the benefits of data-plane in the
> short term.
