Re: [PATCH 09/13] qemu-io: add -t to run an aio request in a named iothread

[Eric Blake <eblake@redhat.com>]

On Wed, Sep 30, 2026 at 06:49:55PM +0200, Denis V. Lunev wrote:
> From: Denis V. Lunev <den@openvz.org>
> 
> A multiqueue virtio-blk drives one node from several AioContexts at
> once, and nothing in the test tools could reproduce that shape: every
> qemu-io request ran in the main loop. Filters and drivers that cache
> state across a yield have therefore never been exercised the way
> production runs them.

Useful addition!

> 
> blk_aio_preadv() and friends enter the request coroutine in the
> caller's context, so submitting from a bottom half scheduled on an
> iothread is all it takes to put the request on that thread. The node
> itself stays where it is, which is what virtio-blk does with the
> queues it maps to iothreads other than the first.
> 
> aio_read and aio_write promise their caller that the request has been
> submitted by the time they return, so wait for the bottom half to run
> before returning. That also makes a test deterministic: entering a
> coroutine from its own context runs it until it yields, so by the time
> the command returns the request has reached, or blocked at, whatever
> the test set up for it.
> 
> Signed-off-by: Denis V. Lunev <den@openvz.org>
> CC: Kevin Wolf <kwolf@redhat.com>
> CC: Hanna Reitz <hreitz@redhat.com>
> CC: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
> CC: Andrey Drobyshev <andrey.drobyshev@virtuozzo.com>
> ---
>  qemu-io-cmds.c | 111 ++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 101 insertions(+), 10 deletions(-)
> 
> diff --git a/qemu-io-cmds.c b/qemu-io-cmds.c
> index aa795fd87a..ce5bb53f4e 100644
> --- a/qemu-io-cmds.c
> +++ b/qemu-io-cmds.c

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.
Virtualization:  qemu.org | libguestfs.org