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

[Oleksandr <olekstysh@xxxxxxxxx>]

Hello Juergen


On 28.04.22 21:03, Oleksandr Tyshchenko wrote:
> On Thu, Apr 28, 2022 at 11:28 AM Juergen Gross <jgross@xxxxxxxx 
> <mailto:jgross@xxxxxxxx>> 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@xxxxxxxx
>     <mailto:jgross@xxxxxxxx>>
>     ---
>      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];
(END)



>                     } else {
>                             to_copy = XEN_NUM_GREFS_PER_PAGE;
>                             page_dir->gref_dir_next_page =
>     buf->grefs[i + 1];
>     -- 
>     2.34.1
>
>
>
> --
> Regards,
>
> Oleksandr Tyshchenko
--
Regards,

Oleksandr Tyshchenko