Re: [PATCH 1/2] viridian: remove implicit limit of 64 VPs per partition

[Jan Beulich <jbeulich@xxxxxxxx>]

On 08.01.2021 12:27, Igor Druzhinin wrote:
> On 08/01/2021 09:19, Jan Beulich wrote:
>> On 08.01.2021 01:46, Igor Druzhinin wrote:
>>> --- a/tools/libs/light/libxl_x86.c
>>> +++ b/tools/libs/light/libxl_x86.c
>>> @@ -336,7 +336,7 @@ static int hvm_set_viridian_features(libxl__gc *gc, 
>>> uint32_t domid,
>>>          LOG(DETAIL, "%s group enabled", 
>>> libxl_viridian_enlightenment_to_string(v));
>>>  
>>>      if (libxl_bitmap_test(&enlightenments, 
>>> LIBXL_VIRIDIAN_ENLIGHTENMENT_BASE)) {
>>> -        mask |= HVMPV_base_freq;
>>> +        mask |= HVMPV_base_freq | HVMPV_no_vp_limit;
>>
>> Could you clarify the point of having the new control when at
>> the guest config level there's no way to prevent it from
>> getting enabled (short of disabling Viridian extensions
>> altogether)?
> 
> If you migrate in from a host without this code (previous version
> of Xen)- you will keep the old behavior for the guest thus preserving
> binary compatibility.

Keeping thing compatible like this is clearly a requirement. But
is this enough? As soon as the guest reboots, it will see differing
behavior, won't it?

Jan