Re: [Xen-devel] Guidelines for new PV protocol submission

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 20/01/15 12:47, Roger Pau Monné wrote:
> Hello,
> 
> I should probably have done this earlier because I've been aware of this
> issue for a long time (since I've started dealing with the PV blk protocol).
> 
> The current way to describe PV protocols in Xen is very inefficient
> IMHO. Using C structs as "the description" of a binary protocol seems
> very wrong, specially taking into account that different ABIs can
> generate different layouts for the same C struct. This is for example a
> problem in the PV blk protocol, since the binary layout of the
> structures change depending on the bitness.
> 
> In order to avoid this, I would like to request that any new PV protocol
> that's added to Xen be described in binary terms, just like it's
> normally done with other protocols. As a reference see for example the
> following section from the TCP RFC:
> 
> http://tools.ietf.org/html/rfc793#page-15
> 
> I think this is both more easy to understand and removes the bitness
> problem of using C structs.
> 
> Then each user of this protocol could define it's own set of structures
> that would map to the binary layout, which should be almost trivial.
> There would be no problem with using __packed or similar gcc'isms as
> each implementation could choose the more convenient way to represent
> this layout internally.

+1

We did this for migration v2 protocol[1] and I agree that this is a much
better way of specifying binary protocols.

David

[1] http://xenbits.xen.org/people/andrewcoop/domain-save-format-F.pdf

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