Re: [Xen-devel] [PATCH] netif.h: Document xen-net{back, front} multi-queue feature

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Mon, 2014-02-17 at 18:01 +0000, Andrew J. Bennieston wrote:
> From: "Andrew J. Bennieston" <andrew.bennieston@xxxxxxxxxx>
> 
> Document the multi-queue feature in terms of XenStore keys to be written
> by the backend and by the frontend.
> 
> Signed-off-by: Andrew J. Bennieston <andrew.bennieston@xxxxxxxxxx>
> ---
>  xen/include/public/io/netif.h |   21 +++++++++++++++++++++
>  1 file changed, 21 insertions(+)
> 
> diff --git a/xen/include/public/io/netif.h b/xen/include/public/io/netif.h
> index d7fb771..90be2fc 100644
> --- a/xen/include/public/io/netif.h
> +++ b/xen/include/public/io/netif.h
> @@ -69,6 +69,27 @@
>   */
>  
>  /*
> + * Multiple transmit and receive queues:
> + * If supported, the backend will write "multi-queue-max-queues" 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.
> + *
> + * Queues replicate the shared rings and event channels, and
> + * "feature-split-event-channels" is required when using multiple 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-channel and ring-ref keys, instead writing them under sub-keys 
> having
> + * the name "queue-N" where N is the integer ID of the queue for which those
> + * keys belong. Queues are indexed from zero.

If "feature-split-event-channels" is required then I think what should
be written is queue-N/event-channel-{tx,rx} and
queue-N/{tx,rx}-ring-ref, rather than queue-N/{event-channel,ring-ref}
as the final paragraph sort of implies?

(what a shame we have event-channel-DIR and DIR-ring-ref, oh well!)

Is it required to have the same number of RX and TX queues?

Are there any other properties/behaviours which should be documented,
e.g. relating to the selection of which queue to use for a given frame
(on either TX or RX)? If not and it is up to the relevant end to do what
it wants then I think it would be useful to say so.

> + */
> +
> +/*
>   * "feature-no-csum-offload" should be used to turn IPv4 TCP/UDP checksum
>   * offload off or on. If it is missing then the feature is assumed to be on.
>   * "feature-ipv6-csum-offload" should be used to turn IPv6 TCP/UDP checksum



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