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SubjectRe: [PATCH v2 08/19] xen/shbuf: switch xen-front-pgdir-shbuf to use INVALID_GRANT_REF
From
On 28.04.22 20:03, 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...

Hmm, that's unfortunate.

> https://elixir.bootlin.com/linux/v5.18-rc4/source/include/xen/interface/io/sndif.h#L650
> <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.

Thanks for finding this.


Juergen
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