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[PATCH v2 1/3] x86/viridian: Workaround Hyper-V GP fault writing to MSR
- To: xen-devel@xxxxxxxxxxxxxxxxxxxx
- From: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
- Date: Fri, 4 Sep 2026 15:15:14 +0100
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- Cc: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>, Paul Durrant <paul@xxxxxxx>, Jan Beulich <jbeulich@xxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger@xxxxxxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>
- Delivery-date: Fri, 04 Sep 2026 14:15:38 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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@xxxxxxxxxx>
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
---
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
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