Re: [Embedded-pv-devel] [Xen-devel] [PATCH v13] This is the ABI for the two halves of a para-virtualized sound driver to communicate with each to other.

[Oleksandr Andrushchenko <andr2000@xxxxxxxxx>]

On 11/28/2016 07:12 PM, Julien Grall wrote:
> On 28/11/16 17:11, Andrew Cooper wrote:
> > On 28/11/16 16:59, Julien Grall wrote:
> > > Hi,
> > >
> > > On 28/11/16 15:43, Oleksandr Andrushchenko wrote:
> > > > On 11/28/2016 05:00 PM, Julien Grall wrote:
> > > > > Hi Oleksandr,
> > > > >
> > > > > On 28/11/16 14:56, Oleksandr Andrushchenko wrote:
> > > > > > On 11/28/2016 04:24 PM, Julien Grall wrote:
> > > > > > > Hi Oleksandr,
> > > > > > >
> > > > > > > On 28/11/16 14:12, Oleksandr Andrushchenko wrote:
> > > > > > > > On 11/28/2016 03:27 PM, Jan Beulich wrote:
> > > > > > > > > > + *
> > > > > > > > > > + * gref_dir_next_page - grant_ref_t, reference to the next page
> > > > > > > > > > describing
> > > > > > > > > > + *   page directory. Must be 0 if no more pages in the list.
> > > > > > > If I am not mistaken 0 is a valid grant.
> > > > > > Then I will remove this sentence, anyways BE knows how many grefs
> > > > > > there
> > > > > > are for the buffer size given
> > > > BTW, xen-blkfrint.c:
> > > > #define GRANT_INVALID_REF    0
> > > > this is from where I got "Must be 0 if no more pages in the list."
> > > GRANT_INVALID_REF is internally to Linux and never exposed in the PV
> > > driver. So for me it is implementation details because ref 0 could be
> > > allocated (log dump by Xen):
> > >
> > > (XEN)       -------- active --------       -------- shared --------
> > > (XEN) [ref] localdom mfn      pin          localdom gmfn flags
> > > (XEN) grant-table for remote domain:    2 (v1)
> > > (XEN) [  0]        0 0x99bf35 0x00000001          0 0x039000 0x19
> > > (XEN) [  1]        0 0x99bf33 0x00000001          0 0x039001 0x19
> > Grant reference 0 is reserved in the ABI for the paravirtual console.
> Oh, so considering grant ref 0 as invalid in a PV protocol would be fine?
Just to summarize: strictly speaking, grant reference 0 is valid, but
never exposed to a PV driver, because of the fact that it is already in
use/reserved for the PV console. Taking into account this fact we can
assume that 0 is GRANT_INVALID_REF and can be used in PV
drivers just like xen-blkfront.c
does.

I will add a note on this in the protocol
> Cheers,

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