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

[Igor Druzhinin <igor.druzhinin@xxxxxxxxxx>]

On 08/01/2021 13:17, Jan Beulich wrote:
> 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?

Yes, that's what I was expecting - after reboot it should be fine.
Hence I put this option into the default set - I'd expect no limit to be
applied in the first place.

Igor