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

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 <[email protected]>
---
 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;
+
         /* Vectors must be in the range 0x10-0xff inclusive */
         new.as_uint64 = val;
         if ( new.vector < 0x10 )
-- 
2.53.0