Re: [PATCH] public/gnttab: relax v2 recommendation

[Julien Grall <julien@xxxxxxx>]

Hi Jan,

On 29/04/2021 14:10, Jan Beulich wrote:
> With there being a way to disable v2 support, telling new guests to use
> v2 exclusively is not a good suggestion.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>
> --- a/xen/include/public/grant_table.h
> +++ b/xen/include/public/grant_table.h
> @@ -121,8 +121,10 @@ typedef uint32_t grant_ref_t;
>    */
>   
>   /*
> - * Version 1 of the grant table entry structure is maintained purely
> - * for backwards compatibility.  New guests should use version 2.
> + * Version 1 of the grant table entry structure is maintained largely for
> + * backwards compatibility.  New guests are recommended to support using
> + * version 2 to overcome version 1 limitations, but to be able to fall back
> + * to version 1.
v2 is not supported on Arm and I don't see it coming anytime soon. 
AFAIK, Linux will also not use grant table v2 unless the guest has a 
address space larger than 44 (?) bits.
I can't remember why Linux decided to not use it everywhere, but this is 
a sign that v2 is not always desired.
So I think it would be better to recommend new guest to use v1 unless 
they hit the limitations (to be details).
Cheers,

--
Julien Grall