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Re: [Qemu-devel] [PATCH 3/3] virtio-scsi: Forbid devices with different

From: Alberto Garcia
Subject: Re: [Qemu-devel] [PATCH 3/3] virtio-scsi: Forbid devices with different iothreads sharing a blockdev
Date: Mon, 28 Jan 2019 16:46:42 +0100
User-agent: Notmuch/0.18.2 (http://notmuchmail.org) Emacs/24.4.1 (i586-pc-linux-gnu)

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?


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