linux-kernel - Re: [Xen-devel] [PATCH v3 2/9] xen-block: add document for mutli hardware queues/rings

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Message-ID: <560F196B.2090504@oracle.com>
Date:	Sat, 03 Oct 2015 07:55:23 +0800
From:	Bob Liu <bob.liu@...cle.com>
To:	Roger Pau Monné <roger.pau@...rix.com>
CC:	Wei Liu <wei.liu2@...rix.com>, xen-devel@...ts.xen.org,
	hch@...radead.org, felipe.franciosi@...rix.com,
	rafal.mielniczuk@...rix.com, linux-kernel@...r.kernel.org,
	jonathan.davies@...rix.com, axboe@...com, david.vrabel@...rix.com,
	avanzini.arianna@...il.com, boris.ostrovsky@...cle.com
Subject: Re: [Xen-devel] [PATCH v3 2/9] xen-block: add document for mutli
 hardware queues/rings


On 10/03/2015 12:22 AM, Roger Pau Monné wrote:
> El 02/10/15 a les 18.12, Wei Liu ha escrit:
>> On Fri, Oct 02, 2015 at 06:04:35PM +0200, Roger Pau Monné wrote:
>>> El 05/09/15 a les 14.39, Bob Liu ha escrit:
>>>> Document multi queues/rings of xen-block.
>>>>
>>>> Signed-off-by: Bob Liu <bob.liu@...cle.com>
>>>
>>> As said by Konrad, you should send this against the Xen public headers
>>> also (or even before). I have a comment below.
>>>

Sure, I'll do that and also rebase this series after get more comments.

>>>> ---
>>>>  include/xen/interface/io/blkif.h |   32 ++++++++++++++++++++++++++++++++
>>>>  1 file changed, 32 insertions(+)
>>>>
>>>> diff --git a/include/xen/interface/io/blkif.h b/include/xen/interface/io/blkif.h
>>>> index c33e1c4..b453b70 100644
>>>> --- a/include/xen/interface/io/blkif.h
>>>> +++ b/include/xen/interface/io/blkif.h
>>>> @@ -28,6 +28,38 @@ typedef uint16_t blkif_vdev_t;
>>>>  typedef uint64_t blkif_sector_t;
>>>>  
>>>>  /*
>>>> + * Multiple hardware queues/rings:
>>>> + * If supported, the backend will write the key "multi-queue-max-queues" to
>>>> + * the directory for that vbd, and set its value to the maximum supported
>>>> + * number of queues.
>>>> + * Frontends that are aware of this feature and wish to use it can write the
>>>> + * key "multi-queue-num-queues", set to the number they wish to use, which
>>>> + * must be greater than zero, and no more than the value reported by the backend
>>>> + * in "multi-queue-max-queues".
>>>> + *
>>>> + * For frontends requesting just one queue, the usual event-channel and
>>>> + * ring-ref keys are written as before, simplifying the backend processing
>>>> + * to avoid distinguishing between a frontend that doesn't understand the
>>>> + * multi-queue feature, and one that does, but requested only one queue.
>>>> + *
>>>> + * Frontends requesting two or more queues must not write the toplevel
>>>> + * event-channeland ring-ref keys, instead writing those keys under sub-keys
>>>> + * having the name "queue-N" where N is the integer ID of the queue/ring for
>>>> + * which those keys belong. Queues are indexed from zero.
>>>> + * For example, a frontend with two queues must write the following set of
>>>> + * queue-related keys:
>>>> + *
>>>> + * /local/domain/1/device/vbd/0/multi-queue-num-queues = "2"
>>>> + * /local/domain/1/device/vbd/0/queue-0 = ""
>>>> + * /local/domain/1/device/vbd/0/queue-0/ring-ref = "<ring-ref>"
>>>> + * /local/domain/1/device/vbd/0/queue-0/event-channel = "<evtchn>"
>>>> + * /local/domain/1/device/vbd/0/queue-1 = ""
>>>> + * /local/domain/1/device/vbd/0/queue-1/ring-ref = "<ring-ref>"
>>>> + * /local/domain/1/device/vbd/0/queue-1/event-channel = "<evtchn>"
>>>
>>> AFAICT, it's impossible by design to use multiple queues together with
>>> multipage rings, is that right?
>>>
>>
>> As far as I can tell, these two features are not inherently coupled.
>> Whether you want to make (by design) them coupled together or not is
>> another matter. :-)
> 
> I haven't looked at the implementation yet, but some mention of whether
> multipage-rings are allowed with multiqueue would be good. For example
> if both can indeed be used in conjunction I would mention:
> 
> If multi-page rings are also used, the format of the grant references
> will be:
> 
> /local/domain/1/device/vbd/0/queue-0/ring-ref0 = "<ring-ref0>"
> /local/domain/1/device/vbd/0/queue-0/ring-ref1 = "<ring-ref1>"
> /local/domain/1/device/vbd/0/queue-0/ring-ref2 = "<ring-ref2>"
> [...]
> 

True, and this is already supported. I'll update the document next version.

-- 
Regards,
-Bob
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