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Re: [PATCH] x86/hvm: Clamp Viridian features to known set
- To: Ross Lagerwall <ross.lagerwall@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 24 Sep 2026 15:08:14 +0200
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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 13:08:22 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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.
Jan
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