Re: [Xen-devel] [Patch 3/7] pvSCSI driver

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 28/2/08 09:23, "Ian Pratt" <Ian.Pratt@xxxxxxxxxxxxx> wrote:

>> In fact, previous version of pvSCSI driver used 2 rings for frontend
>> to backend and backend to frontend communication respectively. The
>> backend also queued requests from frontend and released the ring
>> immediately. This may be very similer concept to the Netchannel2.
> 
> Cool, that sounds better. Did you still have fixed length command
> structs, or allow variable length messages? (I'm very keen we use the
> latter)

The netchannels2 model is 16-byte granularity 'cells' in the comms rings. A
descriptor can span multiple cells and its length is implicitly known from
its type (which resides in the first byte of the first cell). Individual
cells do not wrap in the ring (since the ring is a multiple of 16 bytes in
size) but multi-cell descriptors may wrap. Further, netchannel2 allows
descriptors to be chained into longer messages. I think Jose is going to use
the msb of the descriptor-identifier byte for this purpose.

We've toed-and-froed on details of this protocol a few times, so I've cc'ed
Jose in case the current state of the design is different from what I
believe.

 -- Keir



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