Re: [PATCH v5] x86: make Viridian support optional

[Alejandro Vallejo <alejandro.garciavallejo@xxxxxxx>]

On Wed Oct 15, 2025 at 7:58 AM CEST, Demi Marie Obenour wrote:
> On 10/13/25 06:01, Alejandro Vallejo wrote:
>> On Wed Oct 8, 2025 at 6:04 PM CEST, Jan Beulich wrote:
>>> On 30.09.2025 14:52, 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 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.
>>>
>>> What is "paravirtualized I/O" about in this context?
>> 
>> Hypervisor-assisted IPIs, TLB flushes, etc. Or so I understood back when I 
>> said
>> that looked ok. I see there could be confusion with Xen PV device protocols,
>> but as far as the user of the help message is concerned it makes no 
>> difference.
>> 
>> One could even remove the examples and leave it as "... for Hyper-V guests. 
>> This
>> is to...". They are truly inconsequential.
>> 
>> All that matters is that (modern) Windows won't run without it, and that it
>> provides some indeterminate hypervisor-provided assists to try to reduce some
>> virtualization overheads.
>
> Qubes OS doesn't expose Viridian at all, which is why it wasn't
> vulnerable to XSA-472.  It still runs Windows guests just fine.

Can you run Windows 11?

I don't remember which, but I do know some versions of Windows refuse to boot
if they determine they are virtualised and don't see the mandatory parts of
the TLFS.

If 11 works, maybe Windows Server?

Cheers,
Alejandro