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

[David Vrabel <david.vrabel@xxxxxxxxxx>]

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?

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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