Re: [Qemu-devel] [PATCH v5] vhost-user: add multi queue support

[Martin Kletzander <mkletzan@redhat.com>]

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On Thu, Jul 09, 2015 at 02:24:51PM +0200, Maxime Leroy wrote:
>Hi Martin, Michael,
>
>On Thu, Jul 9, 2015 at 2:00 PM, Martin Kletzander <mkletzan@redhat.com> wrote:
>> On Thu, Jul 09, 2015 at 10:06:35AM +0300, Michael S. Tsirkin wrote:
>>>
>>> On Thu, Jul 09, 2015 at 12:00:59AM +0200, Maxime Leroy wrote:
>>>>
>>>> Hi Michael,
>>>>
>>>> On Wed, Jul 8, 2015 at 4:29 PM, Michael S. Tsirkin <mst@redhat.com>
>>>> wrote:
>>>> > On Thu, May 28, 2015 at 09:23:06AM +0800, Ouyang Changchun wrote:
>>>> >> Based on patch by Nikolay Nikolaev:
>>>> >> Vhost-user will implement the multi queue support in a similar way
>>>> >> to what vhost already has - a separate thread for each queue.
>>>> >> To enable the multi queue functionality - a new command line parameter
>>>> >> "queues" is introduced for the vhost-user netdev.
>>>> >>
>>>> >> Signed-off-by: Nikolay Nikolaev <n.nikolaev@virtualopensystems.com>
>>>> >> Signed-off-by: Changchun Ouyang <changchun.ouyang@intel.com>
>>>> >
>>>> > So testing turned up a significant issue with the protocol extension in
>>>> > this
>>>> > one.  Specifically, remote has no idea how many queues guest actually
>>>> > wants to use (it's dynamic, guest changes this at any time).
>>>> > We need support for enabling and disabling queues dynamically.
>>>> >
>>>> > Given we are past hard freeze, and given no one uses this yet
>>>> > (dpdk upstream did not merge supporting protocol),
>>>> > I think the best thing to do is to disable this functionality for 2.4.
>>>> > I will send a patch to do this shortly.
>>>>
>>>> You are making a wrong statement, we already use multiqueue for
>>>> vhost-user and we expected to have this support officially integrated
>>>> in qemu 2.4.
>>>>
>>>> Libvirt 1.2.17 has been released with multiqueue support for
>>>> vhost-user.
>>>> (http://libvirt.org/git/?p=libvirt.git;a=commit;h=366c22f2bcf1ddb8253c123f93fd18d1ba9eacd6)
>>>> It checks against the version of qemu (i.e. 2.4)  to know if
>>>> multiqueue is supported or not by qemu.
>>>>
>>>> (http://libvirt.org/git/?p=libvirt.git;a=commitdiff;h=7971723b985b9adc27122a3503e7ab38ced2b57f;hp=e7f5510ef2d28ca0ae0ed5751b1fd3018130d6c1)
>>>
>>>
>>> Ouch. Just another example showing how version checks are evil.
>>> We don't want to break libvirt, I agree.
>>>
>>
>> Yes, exactly.  Unfortunately if QEMU doesn't expose it in any way we
>> don't have many more options.
>>
>>> I think what we can do is accept the command line created
>>> by libvirt, just ignore it and use a single queue only.
>>>
>>
>> Anyway, I think it would be pretty OK to disable it *if and only if*
>> you error out with a sensible error message (e.g. "multiple queues are
>> not supported yet").
>
>I consider that accepting the queue parameter for vhost-user but only
>creates a single queue is a bug.
>Unfortunately I don't think we have many solution here for libvirt 1.2.17.0
>
>Agree with Martin, at least, we should display an error message.
>
>>
>> And from 2.5 and next libvirt release we can fix this properly
>> (QEMU - exposing the capability and libvirt - checking for it).
>
>is it possible to backport this fix in the branch 1.2.17 of libvirt ?
>

Yes, sure, that's what maintenance branches are for ;)

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