Re: [Xen-devel] [PATCH V2] netif.h: Document xen-net{back, front} multi-queue feature

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

Sorry for the delay replying to this.

On Mon, 2014-02-24 at 14:28 +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>
> 
> ---
> V2: Improve documentation based on comments about areas which were unclear.
> 
> ---
>  xen/include/public/io/netif.h |   29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/xen/include/public/io/netif.h b/xen/include/public/io/netif.h
> index d7fb771..5d98734 100644
> --- a/xen/include/public/io/netif.h
> +++ b/xen/include/public/io/netif.h
> @@ -69,6 +69,35 @@
>   */
>  
>  /*
> + * 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.

In each case they write to their respective directories in xenstore.
Perhaps that's stating the obvious though.

> + *
> + * Queues replicate the shared rings and event channels, and
> + * "feature-split-event-channels" may be used when using multiple queues.

Do you mean s/may/must/ here? (this is my only significant comment on
this patch, the rest are really minor nits)

> + * Each queue consists of one shared ring pair, i.e. there must be the same
> + * number of tx and rx rings.
> + *
> + * 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 (or event-channel-{tx,rx}) and {tx,rx}-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.

and following on from that the "them" which must be written under the
sub-keys could perhaps be spelled out otherwise the prior reference
event-channel or event-channel-{tx,rx} makes things a little ambiguous.

>  Queues 
> + * are indexed from zero.
> + *
> + * Mapping of packets to queues is considered to be a function of the
> + * transmitting system (backend or frontend) and is not negotiated
> + * between the two. Guests are free to transmit packets on any queue
> + * they choose, provided it has been set up correctly.

Perhaps stating the obvious again but "...and must be prepared to
receive a packet on any ring"?

Ian.


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