Re: [XEN][PATCH v3] x86: make Viridian support optional

[Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>]

On Thu Sep 18, 2025 at 7:13 PM CEST, Grygorii Strashko wrote:
>
>
> On 18.09.25 18:19, Jan Beulich wrote:
>> On 18.09.2025 17:15, Grygorii Strashko wrote:
>>> On 16.09.25 16:41, Grygorii Strashko wrote:
>>>> --- a/xen/arch/x86/hvm/Kconfig
>>>> +++ b/xen/arch/x86/hvm/Kconfig
>>>> @@ -62,6 +62,16 @@ config ALTP2M
>>>>    
>>>>              If unsure, stay with defaults.
>>>>    
>>>> +config HVM_VIRIDIAN
>>>> +  bool "Hyper-V enlightenments for guests" if EXPERT
>>>> +  default y
>>>> +  help
>>>> +    Support optimizations for Hyper-V guests such as faster hypercalls,
>>>> +    efficient timer and interrupt handling, and enhanced paravirtualized
>>>> +    I/O. This is to improve performance and compatibility of Windows VMs.
>>>> +
>>>> +    If unsure, say Y.
>>>> +
>>>
>>> Actually there is a question for x86 Experts -
>>> Does it make sense to have HVM_VIRIDIAN enabled without enabled 
>>> AMD_SVM/INTEL_VMX Virtualization extensions?
>> 
>> It makes as much or as little sense as HVM=y with both of the ones you 
>> mention
>> turned off. Iirc Andrew in particular wanted to permit such configurations, 
>> to
>> allow to prove the (abstract) correctness of building them, even if the
>> resulting hypervisor may be of little use.
>
> I've been thinking about adding "depends on AMD_SVM || INTEL_VMX"
> to cleanly note dependency.

If anything, it would be CONFIG_HVM. Pretty sure one can't sanely implement
minimal viridian fully on PV. Certain extensions assume some degree of hardware
emulation.

> It's very hard to understand dependencies within x86 code :(

They are very sparsely documented, you're not wrong.

Cheers,
Alejandro