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    SubjectRe: [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@oracle.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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