Re: [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@gmail.com>]

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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