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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.



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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