From mboxrd@z Thu Jan  1 00:00:00 1970
Received: from eggs.gnu.org ([209.51.188.92]:60323)
	by lists.gnu.org with esmtp (Exim 4.71)
	(envelope-from <berto@igalia.com>) id 1go9HI-0002Bv-A9
	for qemu-devel@nongnu.org; Mon, 28 Jan 2019 10:56:52 -0500
Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71)
	(envelope-from <berto@igalia.com>) id 1go97Y-0002jW-CR
	for qemu-devel@nongnu.org; Mon, 28 Jan 2019 10:46:49 -0500
From: Alberto Garcia <berto@igalia.com>
In-Reply-To: <20190128145743.GC5756@localhost.localdomain>
References: <cover.1548171741.git.berto@igalia.com>
	<6ad371df66eecffccce29a0bd14695f40edd6120.1548171741.git.berto@igalia.com>
	<735410dc-a63e-6a28-4526-2caa810a797d@redhat.com>
	<w518szbmmw9.fsf@maestria.local.igalia.com>
	<b3e3cb90-32a6-77b0-1c5b-8281181c8ae8@redhat.com>
	<w5136pjmjrp.fsf@maestria.local.igalia.com>
	<20190124091222.GB4601@localhost.localdomain>
	<w51d0ogsv1y.fsf@maestria.local.igalia.com>
	<20190128145743.GC5756@localhost.localdomain>
Date: Mon, 28 Jan 2019 16:46:42 +0100
Message-ID: <w517eeoss2l.fsf@maestria.local.igalia.com>
MIME-Version: 1.0
Content-Type: text/plain
Subject: Re: [Qemu-devel] [PATCH 3/3] virtio-scsi: Forbid devices with
 different iothreads sharing a blockdev
List-Id: <qemu-devel.nongnu.org>
List-Unsubscribe: <https://lists.nongnu.org/mailman/options/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=unsubscribe>
List-Archive: <http://lists.nongnu.org/archive/html/qemu-devel/>
List-Post: <mailto:qemu-devel@nongnu.org>
List-Help: <mailto:qemu-devel-request@nongnu.org?subject=help>
List-Subscribe: <https://lists.nongnu.org/mailman/listinfo/qemu-devel>,
	<mailto:qemu-devel-request@nongnu.org?subject=subscribe>
To: Kevin Wolf <kwolf@redhat.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>, qemu-devel@nongnu.org, qemu-block@nongnu.org, Max Reitz <mreitz@redhat.com>, Stefan Hajnoczi <stefanha@redhat.com>

On Mon 28 Jan 2019 03:57:43 PM CET, Kevin Wolf wrote:

>> > I think the proper solution on the block layer level would be that
>> > AioContext is managed per BdrvChild and only BdrvChild objects with
>> > the same AioContext can be attached to the same node.
>> > bdrv_set_aio_context() would then fail if another parent is already
>> > using the node.
>> 
>> How would that solve the virtio-blk problem, though? :-?
>
> It wouldn't, but it would make clear that bdrv_set_aio_context() can
> fail and we'd have to handle the problem exactly there.

Yes, that sound like the proper solution.

> The only other option I see is downgrading to non-dataplane mode, but
> this would have to be silent because during machine reset we have no
> way to report errors.

Yes, I actually had a patch doing that, but as you said the user has no
way of knowing that it went wrong.

If you have 2 or 3 virtio-blk devices with different iothreads using the
same BDS then after the guest reboot only one would succeed, but you
can't see which one, or can you?

Berto