Re: [PATCH v1 1/2] x86/viridian: Workaround Hyper-V GP fault writing to MSR

[Ross Lagerwall <ross.lagerwall@citrix.com>]

On 9/3/26 12:14 PM, Jan Beulich wrote:
> On 03.09.2026 12:55, Ross Lagerwall wrote:
>> On 9/3/26 11:44 AM, Jan Beulich wrote:
>>> On 28.08.2026 15:11, Ross Lagerwall wrote:
>>>> The Viridian spec requires the vector to be >= 0x10 when writing to the
>>>> SINTx MSR, otherwise it should #GP fault.
>>>> The spec-defined initial value is 0x0000000000010000, i.e. the vector is
>>>> 0. On startup, for some reason Windows 11 Hyper-V tries to set the MSR to
>>>> the spec-defined initial value and since the vector is 0, it GP faults.
>>>> To workaround this, treat the write as a no-op if the value is
>>>> unchanged.
>>>>
>>>> Signed-off-by: Ross Lagerwall <ross.lagerwall@citrix.com>
>>>
>>> Acked-by: Jan Beulich <jbeulich@suse.com>
>>>
>>> I expect that despite the (valid) absence of a Fixes: tag this will want
>>> backporting.
>>
>> It should be fine to backport, though in general I think backporting nested
>> virt fixes at this point is not needed as there will be too many changes to go
>> into the stable releases for it to reach a usable state.
> 
> How is this nested-virt related? It looks to affect Win11 when run on bare-
> metal Xen?

The regular Windows 11 kernel doesn't have this behaviour on startup, only the
Windows 11 Hyper-V kernel (and possibly other Windows versions too) so you
wouldn't encounter this behaviour unless using nested-virt.

Ross