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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 Acked-by: Jan Beulich --- In v2: Added a comment to clarify xen/arch/x86/hvm/viridian/synic.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/xen/arch/x86/hvm/viridian/synic.c b/xen/arch/x86/hvm/viridian/synic.c index e6cba7548f1b..75b004440e58 100644 --- a/xen/arch/x86/hvm/viridian/synic.c +++ b/xen/arch/x86/hvm/viridian/synic.c @@ -157,6 +157,14 @@ int viridian_synic_wrmsr(struct vcpu *v, uint32_t idx, uint64_t val) if ( !(viridian_feature_mask(d) & HVMPV_synic) ) return X86EMUL_EXCEPTION; + /* + * Windows 11 Hyper-V (26H1) has been seen to write this MSR to + * its default value, despite not being a spec compliant value. + * Tolerate writes which have no change in value. + */ + if ( val == vs->as_uint64 ) + break; + /* Vectors must be in the range 0x10-0xff inclusive */ new.as_uint64 = val; if ( new.vector < 0x10 ) -- 2.53.0