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Re: [PATCH v1 1/2] x86/viridian: Workaround Hyper-V GP fault writing to MSR
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
- Date: Thu, 3 Sep 2026 12:26:40 +0100
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- Cc: Paul Durrant <paul@xxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Roger Pau Monné <roger@xxxxxxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 03 Sep 2026 11:26:56 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 9/3/26 12:14 PM, Jan Beulich wrote:
On 03.09.2026 12:55, Ross Lagerwall wrote:
On 9/3/26 11:44 AM, Jan Beulich wrote:
On 28.08.2026 15:11, 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@xxxxxxxxxx>
Acked-by: Jan Beulich <jbeulich@xxxxxxxx>
I expect that despite the (valid) absence of a Fixes: tag this will want
backporting.
It should be fine to backport, though in general I think backporting nested
virt fixes at this point is not needed as there will be too many changes to go
into the stable releases for it to reach a usable state.
How is this nested-virt related? It looks to affect Win11 when run on bare-
metal Xen?
The regular Windows 11 kernel doesn't have this behaviour on startup, only the
Windows 11 Hyper-V kernel (and possibly other Windows versions too) so you
wouldn't encounter this behaviour unless using nested-virt.
Ross
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