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Re: [PATCH] x86/hvm: Clamp Viridian features to known set
- To: Jan Beulich <jbeulich@xxxxxxxx>
- From: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
- Date: Thu, 24 Sep 2026 16:30:37 +0100
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- Cc: Roger Pau Monné <roger@xxxxxxxxxxxxxx>, Teddy Astie <teddy.astie@xxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 24 Sep 2026 15:30:51 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 9/24/26 2:08 PM, Jan Beulich wrote:
On 24.09.2026 14:56, Ross Lagerwall wrote:
On 9/24/26 12:20 PM, Andrew Cooper wrote:
On 24/09/2026 12:03 pm, Ross Lagerwall wrote:
Instead of returning EINVAL on an unknown feature bit, clamp to the
known feature set.
Sorry, but no.
This is equivalent to saying "I've got a VM using AVX512" and Xen saying
"ok, I'll ignore that safety check and let you run on non-AVX512 capable
hardware".
Requesting a feature that Xen doesn't know about is a hard error.
Truncating features out like this will cause a guest using those
features to malfunction.
It is a bug that this was expressed as an HVM Param in the first place.
It should be part of CPU Policy, and it's on a TODO list.
But isn't this the same thing the CPU Policy code does? In
recalculate_cpuid_policy(), it silently clamps the toolstack's choices to the
max featureset.
Which is behavior that, if I'm not mistaken, is supposed to go away. Such
requests are intended to instead fail, down the road.
OK, if that is undesirable behaviour and going to change in future I'll retract
this patch.
Ross
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