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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

This reuse looks erroneous on behalf of blkfront.

~Andrew

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