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Date: Mon, 2 May 2022 16:52:29 +0300
From: Oleksandr <olekstysh@...il.com>
To: Juergen Gross <jgross@...e.com>,
xen-devel <xen-devel@...ts.xenproject.org>,
Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc: Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Stefano Stabellini <sstabellini@...nel.org>
Subject: Re: [PATCH v2 08/19] xen/shbuf: switch xen-front-pgdir-shbuf to use
INVALID_GRANT_REF
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@...e.com
>>> <mailto:jgross@...e.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@...e.com
>>> <mailto:jgross@...e.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@...vator-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
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