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

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 28/08/2026 2:11 pm, 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>
> ---
>  xen/arch/x86/hvm/viridian/synic.c | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/xen/arch/x86/hvm/viridian/synic.c b/xen/arch/x86/hvm/viridian/synic.c
> index e6cba7548f1b..37ddff201a6b 100644
> --- a/xen/arch/x86/hvm/viridian/synic.c
> +++ b/xen/arch/x86/hvm/viridian/synic.c
> @@ -157,6 +157,9 @@ int viridian_synic_wrmsr(struct vcpu *v, uint32_t idx, uint64_t val)
>          if ( !(viridian_feature_mask(d) & HVMPV_synic) )
>              return X86EMUL_EXCEPTION;
>  
> +        if ( val == vs->as_uint64 )
> +            break;

This needs a code comment, or it's something which will be deleted in a
few years time when someone wonders why it's present.

/* Windows 11 Hyper-V (Summer 2026) has been seen to write this MSR to
it's default value, despite not being a spec compliant value.  Tolerate
writes which have no change in value. */

~Andrew