Re: [Xen-devel] [PATCH 2/3] public/io/netif.h: document control ring and toeplitz hashing

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: David Vrabel [mailto:david.vrabel@xxxxxxxxxx]
> Sent: 04 January 2016 10:56
> To: Paul Durrant; David Vrabel; xen-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: Tim (Xen.org); Keir (Xen.org); Ian Campbell; Jan Beulich; Ian Jackson
> Subject: Re: [Xen-devel] [PATCH 2/3] public/io/netif.h: document control
> ring and toeplitz hashing
> 
> On 04/01/16 09:37, Paul Durrant wrote:
> >>> + * The layout of the shared page is as follows:
> >>> + *
> >>> + *    0     1     2     3     4     5     6     7  octet
> >>> + * +-----+-----+-----+-----+-----+-----+-----+-----+
> >>> + * |        req_cons       |        req_prod       |
> >>> + * +-----+-----+-----+-----+-----+-----+-----+-----+
> >>> + * |        rsp_cons       |        rsp_prod       |
> >>> + * +-----+-----+-----+-----+-----+-----+-----+-----+
> >>> + * |                                               |
> >>> + * +                                               +
> >>> + * |                      req[1024]                |
> >>> + *                         .
> >>> + *                         .
> >>> + * |                                               |
> >>> + * +-----+-----+-----+-----+-----+-----+-----+-----+
> >>> + * |                                               |
> >>> + * +                                               +
> >>> + * |                      rsp[1024]                |
> >>> + *                         .
> >>> + *                         .
> >>> + * |                                               |
> >>> + * +-----+-----+-----+-----+-----+-----+-----+-----+
> >>
> >> You should use the standard ring format and infrastructure.
> >
> > Is there one for variable message size rings? I didn't find one. I
> > don't want to use the fixed size balanced ring macros for control
> > messages as fixed size messages really aren't appropriate in this case.
> 
> Perhaps union the request/response message types with a uint8_t
> pad[1024] and use this as the request/response type?
> 

The problem is that this places a 1k limit on the message size, which is not 
there in the scheme I'm proposing. I'd rather not bake that limit in if I don't 
have to.

  Paul

> You can use the standard macros like so (to avoid copying the full 1024
> bytes every time):
> 
> hdr = RING_GET_REQUEST(...);
> switch (READ_ONCE(hdr->type)) {
> case FOO:
>     {
>         struct foo foo;
>         RING_COPY_REQUEST(ring, cons, &foo);
>         handle_foo(&foo);
>     }
>     break;
> case ...:
>     ...
> }
> 
> David

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