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From: Andrey Korolyov <andrey@xdel.ru>
Date: Fri, 29 May 2015 14:11:55 +0300
Message-ID: <CABYiri8eTaESP2rf2d1VBvAR4hsahXP5k91xmbmo560PaZ7whg@mail.gmail.com>
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Subject: Re: [Qemu-devel] [PATCH v6 01/13] block: Add op blocker type
	"device IO"
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To: Fam Zheng <famz@redhat.com>
Cc: Kevin Wolf <kwolf@redhat.com>, qemu block <qemu-block@nongnu.org>, jcody@redhat.com, Markus Armbruster <armbru@redhat.com>, "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>, Stefan Hajnoczi <stefanha@redhat.com>, Amit Shah <amit.shah@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>, Max Reitz <mreitz@redhat.com>

On Thu, May 28, 2015 at 3:05 PM, Fam Zheng <famz@redhat.com> wrote:
> On Thu, 05/28 13:19, Paolo Bonzini wrote:
>>
>>
>> On 28/05/2015 13:11, Fam Zheng wrote:
>> > > Whoever uses ioeventfd needs to implement pause/resume, yes---not just
>> > > dataplane, also "regular" virtio-blk/virtio-scsi.
>> > >
>> > > However, everyone else should be okay, because the bottom half runs
>> > > immediately and the big QEMU lock is not released in the meanwhile.  So
>> > > the CPUs have no occasion to run.  This needs a comment!
>> >
>> > I'm not sure. It seems timer callbacks also do I/O, for example
>> > nvme_process_sq().
>>
>> Right, that's also true for USB devices. :(
>>
>> Perhaps we can skip block_job_defer_to_main_loop if not necessary
>> (bs->aio_context == qemu_get_aio_context()).
>
> I think so. It will make dataplane even more specialized but that seems the
> only way to fix the problem at the moment.
>
> Fam
>

Sorry for a potential thread hijack, but I`m curious about the reasons
to not making advertised queue depth for non-passthrough backends an
independent tunable, is there any concerns behind that?