block: Note in which AioContext AIO CBs are called

This doesn’t seem to be specified anywhere, but is something we probably
want to be clear.  I believe it is reasonable to implicitly assume that
callbacks are run in the current thread (unless explicitly noted
otherwise), so codify that assumption.

Some implementations don’t actually fulfill this contract yet.  The next
patches should rectify that.

Note: I don’t know of any user-visible bugs produced by not running AIO
callbacks in the original context.  AIO functionality is generally
mapped to coroutines through the use of bdrv_co_io_em_complete(), which
can run in any AioContext, and will always wake the yielding coroutine
in its original context.  The only benefit here is that running
bdrv_co_io_em_complete() in the original context will make that
aio_co_wake() most likely a simpler qemu_coroutine_enter() instead of
scheduling the wakeup through AioContext.co_schedule_bh.

Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-ID: <20251110154854.151484-17-hreitz@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

include/block/block_int-common.h

@@ -508,7 +508,12 @@ struct BlockDriver {
         BlockDriverState *bs, BlockdevAmendOptions *opts, bool force,
         Error **errp);
 
-    /* aio */
+    /*
+     * AIO
+     * The given completion callback will be run in the same AioContext as the
+     * one in which the AIO function was called.
+     */
+
     BlockAIOCB * GRAPH_RDLOCK_PTR (*bdrv_aio_preadv)(BlockDriverState *bs,
         int64_t offset, int64_t bytes, QEMUIOVector *qiov,
         BdrvRequestFlags flags, BlockCompletionFunc *cb, void *opaque);