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Re: [PATCH v2 2/2] x86/cpuid: Detect null segment behaviour on Zen2 CPUs


  • To: Jan Beulich <jbeulich@xxxxxxxx>, Jane Malalane <jane.malalane@xxxxxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Wed, 8 Sep 2021 13:28:35 +0100
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On 08/09/2021 13:08, Jan Beulich wrote:
> On 08.09.2021 10:19, Jane Malalane wrote:
>> Zen2 CPUs actually have this behaviour, but the CPUID bit couldn't be
>> introduced into Zen2 due to a lack of leaves. So, it was added in a
>> new leaf in Zen3. Nonetheless, hypervisors can synthesize the CPUID
>> bit in software.
> Considering the prior model checks, I understand this isn't all Zen2s?
> No matter what the answer, I'd like to ask that the first sentence
> start with either "All" or "Some" (or something along these lines).
> Which is of course fine to insert while committing, so not need to
> send a v3.

All Zen2's have this behaviour.

The model checks were trying to avoid running the probe on Zen1, but
"model >= 0x30 && mode != 0x50" is error prone.

There are no faults in the probe, and its fast, so running on all Zen1/2
is better than hoping that we got the model list correct.

~Andrew




 


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