[PATCH] gnttab: don't allocate status frame tracking array when "gnttab=max_ver:1"

[PATCH] gnttab: don't allocate status frame tracking array when "gnttab=max_ver:1"

[Jan Beulich]

This array can be large when many grant frames are permitted; avoid
allocating it when it's not going to be used anyway. Do so indirectly
though, by making grant_to_status_frames() return zero in this case.

Signed-off-by: Jan Beulich <jbeulich@suse.com>

--- a/xen/common/grant_table.c
+++ b/xen/common/grant_table.c
@@ -446,11 +446,14 @@ static inline void active_entry_release(
 
 static inline unsigned int grant_to_status_frames(unsigned int grant_frames)
 {
+    if ( opt_gnttab_max_version < 2 )
+        return 0;
     return DIV_ROUND_UP(grant_frames * GRANT_PER_PAGE, GRANT_STATUS_PER_PAGE);
 }
 
 static inline unsigned int status_to_grant_frames(unsigned int status_frames)
 {
+    ASSERT(opt_gnttab_max_version >= 2);
     return DIV_ROUND_UP(status_frames * GRANT_STATUS_PER_PAGE, GRANT_PER_PAGE);
 }
 

Ping: [PATCH] gnttab: don't allocate status frame tracking array when "gnttab=max_ver:1"

[Jan Beulich]

On 05.11.2020 16:55, Jan Beulich wrote:
> This array can be large when many grant frames are permitted; avoid
> allocating it when it's not going to be used anyway. Do so indirectly
> though, by making grant_to_status_frames() return zero in this case.
> 
> Signed-off-by: Jan Beulich <jbeulich@suse.com>

Ping?

> --- a/xen/common/grant_table.c
> +++ b/xen/common/grant_table.c
> @@ -446,11 +446,14 @@ static inline void active_entry_release(
>  
>  static inline unsigned int grant_to_status_frames(unsigned int grant_frames)
>  {
> +    if ( opt_gnttab_max_version < 2 )
> +        return 0;
>      return DIV_ROUND_UP(grant_frames * GRANT_PER_PAGE, GRANT_STATUS_PER_PAGE);
>  }
>  
>  static inline unsigned int status_to_grant_frames(unsigned int status_frames)
>  {
> +    ASSERT(opt_gnttab_max_version >= 2);
>      return DIV_ROUND_UP(status_frames * GRANT_STATUS_PER_PAGE, GRANT_PER_PAGE);
>  }
>  
> 

Re: [PATCH] gnttab: don't allocate status frame tracking array when "gnttab=max_ver:1"

[Julien Grall]

Hi Jan,

On 05/11/2020 15:55, Jan Beulich wrote:
> This array can be large when many grant frames are permitted; avoid
> allocating it when it's not going to be used anyway. 

Given there are not many users of grant-table v2, would it make sense to 
avoid allocating the array until the guest start using grant-table v2?

Cheers,

-- 
Julien Grall

Re: [PATCH] gnttab: don't allocate status frame tracking array when "gnttab=max_ver:1"

[Jan Beulich]

On 02.12.2020 19:31, Julien Grall wrote:
> On 05/11/2020 15:55, Jan Beulich wrote:
>> This array can be large when many grant frames are permitted; avoid
>> allocating it when it's not going to be used anyway. 
> 
> Given there are not many users of grant-table v2, would it make sense to 
> avoid allocating the array until the guest start using grant-table v2?

Hmm, yes, seems possible. Let me give this a try.

Jan