Re: [PATCH] x86/cpu/centaur: Disable X86_FEATURE_FSGSBASE on Zhaoxin C4600

[Borislav Petkov]

On Sat, Feb 28, 2026 at 04:33:18PM -0800, Dave Hansen wrote:
> On 2/28/26 09:37, Yao Zi wrote:
> > Let's disable the feature on this problematic CPU and warn the user
> > about the quirk. x86_model_id is used to match the platform to avoid
> > unexpectedly breaking other CentaurHauls cores with conflicting
> > family/model ID.
> 
> Wait a sec. There are lots of different microarchitectures with the same
> family/model and no other way to identify them but the model id string?
> 
> We've used string in a handful of places, but it's an absolute last
> resort. Are you *sure* there's no stepping or anything?
> 
> I kinda think we should keep this like all the other vendors and keep it
> to model/family/stepping. If the vendor has grouped too many
> non-vulnerable CPUs under that, then ... this is going to be a good
> learning to bring back to the CPU design team.

Except that you'll be punishing perfectly fine CPUs... that is if those other
ones do really support the FSGSBASE set... and frankly, I don't care too
deeply if that particular zoo of models would use a model string if really
necessary.

But, the first thing we should do, is figure out whether the VIA design is
affected the same way.

Lemme CC some more folks who I can dig out from patches touching that area...

All those newly CCed folks, thread starts here:

https://lore.kernel.org/r/20260228173704.62460-1-me@xxxxxxxx

Please have a look and let us know if you can whether VIA CPUs of family 6,
model 15 and use the "CentaurHauls" model string also claim that

CPUID(7)_EBX[0 /* FSGSBASE */] = 1b

but they also #UD on those insns.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette