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

[Yao Zi <me@ziyao.cc>]

On Sat, Feb 28, 2026 at 08:06:15PM +0100, Borislav Petkov wrote:
> On Sat, Feb 28, 2026 at 05:37:04PM +0000, Yao Zi wrote:
> > Zhaoxin C4600, which names itself as CentaurHauls, claims
> > X86_FEATURE_FSGSBASE support in CPUID, while execution of fsgsbase-
> > related instructions fails with #UD exception. This will cause kernel
> > to crash early in current_save_fsgs().
> > 
> > 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.
> 
> Please use passive voice in your commit message: no "we" or "I", etc,
> and describe your changes in imperative mood.
> 
> Also, pls read section "2) Describe your changes" in
> Documentation/process/submitting-patches.rst for more details.
> 
> Also, see section "Changelog" in
> Documentation/process/maintainer-tip.rst

Okay.

> > Cc: stable@vger.kernel.org
> > Signed-off-by: Yao Zi <me@ziyao.cc>
> > ---
> >  arch/x86/kernel/cpu/centaur.c | 25 +++++++++++++++++++++++++
> >  1 file changed, 25 insertions(+)
> > 
> > diff --git a/arch/x86/kernel/cpu/centaur.c b/arch/x86/kernel/cpu/centaur.c
> > index 81695da9c524..3773784ba6a9 100644
> > --- a/arch/x86/kernel/cpu/centaur.c
> > +++ b/arch/x86/kernel/cpu/centaur.c
> > @@ -108,6 +108,29 @@ static void early_init_centaur(struct cpuinfo_x86 *c)
> >  	}
> >  }
> >  
> > +/*
> > + * Zhaoxin C4600 (family 6, model 15) names itself as CentaurHauls, it claims
> > + * X86_FEATURE_FSGSBASE support in CPUID, while executing any fsgsbase-related
> > + * instructions on it results in #UD.
> > + */
> > +static void fixup_zhaoxin_fsgsbase(struct cpuinfo_x86 *c)
> 
> s/fixup/disable/

Okay.

> > +{
> > +	const char *name, *model_names[] = {
> > +		"C-QuadCore C4600"
> > +	};
> 
> Why is this an array with a single string in it?
> 
> > +	int i;
> > +
> > +	for (i = 0; i < ARRAY_SIZE(model_names); i++) {
> 
> So that you can loop once with it?
> 
> Silly.

Though I don't have the conditions to confirm it, it's likely other CPUs
in the same generation of designs from Zhaoxin have similar issues:
their specifications[1] are mostly identical except the core frequency,
thus they're likely the same die. So I leave a loop here to ease latter
additions if necessary. Sorry not to make it clear.

This may be a little farsight. Dave suggests declaring an x86_cpu_id
array and switching to x86_cpu_match(), do you think it's acceptable? Or
should I focus only on the known problematic model and use a simple
if to match Zhaoxin C4600 for now?

> > +		name = model_names[i];
> > +
> > +		if (!strncmp(c->x86_model_id, name, strlen(name))) {
> > +			pr_warn_once("CPU has broken FSGSBASE support\n");
> > +			setup_clear_cpu_cap(X86_FEATURE_FSGSBASE);
> > +			return;
> > +		}
> > +	}
> > +}
> 
> -- 
> Regards/Gruss,
>     Boris.
> 
> https://people.kernel.org/tglx/notes-about-netiquette

Regards,
Yao Zi

[1]: https://www.zhaoxin.com/qt.aspx?nid=3&typeid=90