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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 30.04.2021 10:19, Julien Grall wrote:
> 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.

Yes, as soon as there are frame numbers not representable in 32 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).

IOW you'd prefer s/be able to fall back/default/? I'd be fine that way.

Jan