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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 > --- > 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