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

[Juergen Gross <jgross@suse.com>]

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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.


Juergen

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