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

["Roger Pau Monné" <roger.pau@citrix.com>]

On Sat, Oct 18, 2025 at 09:21:37PM -0400, Demi Marie Obenour wrote:
> On 10/17/25 11:52, Alejandro Vallejo wrote:
> > 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 haven't tried it, but it is documented as working.
> 
> > 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?
> 
> Windows Server is more likely.

FTR, for unrelated reasons I've tested Windows Server 2025 without
Viridian, and it does work, albeit painfully slow.

Regards, Roger.