Re: [PATCH v2 08/19] xen/shbuf: switch xen-front-pgdir-shbuf to use INVALID_GRANT_REF

[Oleksandr <olekstysh@gmail.com>]

On 02.05.22 16:31, Juergen Gross wrote:


Hello Juergen


> On 29.04.22 17:28, Oleksandr wrote:
>>
>> Hello Juergen
>>
>>
>> On 28.04.22 21:03, Oleksandr Tyshchenko wrote:
>>>
>>>
>>> On Thu, Apr 28, 2022 at 11:28 AM Juergen Gross <jgross@suse.com 
>>> <mailto:jgross@suse.com>> wrote:
>>>
>>> Hello Juergen
>>>
>>> [sorry for the possible format issue]
>>>
>>>     Instead of using a private macro for an invalid grant reference use
>>>     the common one.
>>>
>>>     Signed-off-by: Juergen Gross <jgross@suse.com
>>>     <mailto:jgross@suse.com>>
>>>     ---
>>>      drivers/xen/xen-front-pgdir-shbuf.c | 17 ++++-------------
>>>      1 file changed, 4 insertions(+), 13 deletions(-)
>>>
>>>     diff --git a/drivers/xen/xen-front-pgdir-shbuf.c
>>>     b/drivers/xen/xen-front-pgdir-shbuf.c
>>>     index a959dee21134..fa2921d4fbfc 100644
>>>     --- a/drivers/xen/xen-front-pgdir-shbuf.c
>>>     +++ b/drivers/xen/xen-front-pgdir-shbuf.c
>>>     @@ -21,15 +21,6 @@
>>>
>>>      #include <xen/xen-front-pgdir-shbuf.h>
>>>
>>>     -#ifndef GRANT_INVALID_REF
>>>     -/*
>>>     - * FIXME: usage of grant reference 0 as invalid grant reference:
>>>     - * grant reference 0 is valid, but never exposed to a PV driver,
>>>     - * because of the fact it is already in use/reserved by the PV
>>>     console.
>>>     - */
>>>     -#define GRANT_INVALID_REF      0
>>>     -#endif
>>>     -
>>>      /**
>>>       * This structure represents the structure of a shared page
>>>       * that contains grant references to the pages of the shared
>>>     @@ -83,7 +74,7 @@ grant_ref_t
>>>      xen_front_pgdir_shbuf_get_dir_start(struct xen_front_pgdir_shbuf
>>>     *buf)
>>>      {
>>>             if (!buf->grefs)
>>>     -               return GRANT_INVALID_REF;
>>>     +               return INVALID_GRANT_REF;
>>>
>>>             return buf->grefs[0];
>>>      }
>>>     @@ -142,7 +133,7 @@ void xen_front_pgdir_shbuf_free(struct
>>>     xen_front_pgdir_shbuf *buf)
>>>                     int i;
>>>
>>>                     for (i = 0; i < buf->num_grefs; i++)
>>>     -                       if (buf->grefs[i] != GRANT_INVALID_REF)
>>>     +                       if (buf->grefs[i] != INVALID_GRANT_REF)
>>>     gnttab_end_foreign_access(buf->grefs[i], 0UL);
>>>             }
>>>             kfree(buf->grefs);
>>>     @@ -355,7 +346,7 @@ static void backend_fill_page_dir(struct
>>>     xen_front_pgdir_shbuf *buf)
>>>             }
>>>             /* Last page must say there is no more pages. */
>>>             page_dir = (struct xen_page_directory *)ptr;
>>>     -       page_dir->gref_dir_next_page = GRANT_INVALID_REF;
>>>     +       page_dir->gref_dir_next_page = INVALID_GRANT_REF;
>>>      }
>>>
>>>      /**
>>>     @@ -384,7 +375,7 @@ static void guest_fill_page_dir(struct
>>>     xen_front_pgdir_shbuf *buf)
>>>
>>>                     if (grefs_left <= XEN_NUM_GREFS_PER_PAGE) {
>>>                             to_copy = grefs_left;
>>>     -                       page_dir->gref_dir_next_page =
>>>     GRANT_INVALID_REF;
>>>     +                       page_dir->gref_dir_next_page =
>>>     INVALID_GRANT_REF;
>>>
>>>
>>> I faced an issue with testing PV Sound with the current series.
>>>
>>> root@salvator-x-h3-4x2g-xt-domu:~# aplay /media/MoodyLoop.wav
>>> Playing WAVE '/media/MoodyLoop.wav' : Signed 16 bit Little Endian, 
>>> Rate 44100 Hz, Stereo
>>> (XEN) common/grant_table.c:1053:d1v2 Bad ref 0xffffffff for d6
>>>
>>> Here we have an interesting situation. PV Sound frontend uses this 
>>> xen-front-pgdir-shbuf framework. Technically, this patch changes 
>>> page_dir->gref_dir_next_page (reference to the next page describing 
>>> page directory) from 0 to 0xffffffff here.
>>> #define INVALID_GRANT_REF  ((grant_ref_t)-1)
>>>
>>> But according to the protocol (sndif.h), "0" means that there are no 
>>> more pages in the list and the user space backend expects only that 
>>> value. So receiving 0xffffffff it assumes there are pages in the 
>>> list and trying to process...
>>> https://elixir.bootlin.com/linux/v5.18-rc4/source/include/xen/interface/io/sndif.h#L650 
>>>
>>>
>>>
>>> I think, the same is relevant to backend_fill_page_dir() as well.
>>
>>
>> In addition to what I said yesterday:
>>
>> PV Display also uses this xen-front-pgdir-shbuf framework. It's 
>> protocol (displif.h) also mentions the same as sndif.h if the context 
>> of gref_dir_next_page:
>>
>>   * gref_dir_next_page - grant_ref_t, reference to the next page 
>> describing
>>   *   page directory. Must be 0 if there are no more pages in the list.
>>
>>
>> With that local change both PV devices work in my environment.
>>
>> diff --git a/drivers/xen/xen-front-pgdir-shbuf.c 
>> b/drivers/xen/xen-front-pgdir-shbuf.c
>> index fa2921d..ad4a88e 100644
>> --- a/drivers/xen/xen-front-pgdir-shbuf.c
>> +++ b/drivers/xen/xen-front-pgdir-shbuf.c
>> @@ -346,7 +346,7 @@ static void backend_fill_page_dir(struct 
>> xen_front_pgdir_shbuf *buf)
>>          }
>>          /* Last page must say there is no more pages. */
>>          page_dir = (struct xen_page_directory *)ptr;
>> -       page_dir->gref_dir_next_page = INVALID_GRANT_REF;
>> +       page_dir->gref_dir_next_page = 0;
>>   }
>>
>>   /**
>> @@ -375,7 +375,7 @@ static void guest_fill_page_dir(struct 
>> xen_front_pgdir_shbuf *buf)
>>
>>                  if (grefs_left <= XEN_NUM_GREFS_PER_PAGE) {
>>                          to_copy = grefs_left;
>> -                       page_dir->gref_dir_next_page = 
>> INVALID_GRANT_REF;
>> +                       page_dir->gref_dir_next_page = 0;
>>                  } else {
>>                          to_copy = XEN_NUM_GREFS_PER_PAGE;
>>                          page_dir->gref_dir_next_page = buf->grefs[i 
>> + 1];
>
> I think I'll introduce a proper define for that purpose.


I think it would be the best option.



>
>
>
> Juergen

-- 
Regards,

Oleksandr Tyshchenko