Re: [Xen-devel] [DOC v2] Xen transport for 9pfs

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

On 12/12/2016 02:00 PM, Wei Liu wrote:
> On Mon, Dec 05, 2016 at 05:33:23PM -0800, Stefano Stabellini wrote:
> [...]
> > ## Xenstore
> >
> > The frontend and the backend connect via xenstore to exchange
> > information. The toolstack creates front and back nodes with state
> > [XenbusStateInitialising]. The protocol node name is **9pfs**.
> >
> > Multiple rings are supported for each frontend and backend connection.
> It would help if you can state if a specific node is mandatory or
> optional.
>
> > ### Frontend XenBus Nodes
> > ### Backend XenBus Nodes
> [...]
>
> > The producer always notifies the consumer after incrementing **prod**.
> > However in some circumstances the producer is allowed not to notify the
> > consumer, just as a performance improvement, and still maintain
> > correctness. These are the steps to do it: after incrementing *prod*,
> > the producer reads *cons* a second time; if the value is changed from
> > the previous read and it is different from *prod* before the update,
> > then the notification can be avoided. These are the producer steps, with
> > the optimization:
> >
> > - read *prod* (old_prod), *cons* (old_cons) from shared memory
> > - general memory barrier
> > - verify *prod* against local copy (consumer shouldn't change it)
> > - write to array at position *prod* up to *cons*, wrapping around the circular
> >    buffer when necessary
> > - write memory barrier
> > - increase *prod* (new_prod)
> > - general memory barrier
> > - read *cons* (new_cons)
> > - if new_cons == old_cons or new_cons == old_prod, then notify the
> >    consumer
> I think it would be valuable to extract this section to a generic "how
> to write driver" doc. But that's probably something for another day.
I do support this idea which will save lots of time and efforts
for the PV developers...
> Wei.
>
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