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

[Yao Zi <me@ziyao.cc>]

On Sun, Mar 01, 2026 at 06:37:47PM +0000, David Laight wrote:
> On Sun, 1 Mar 2026 09:27:29 +0000
> Yao Zi <me@ziyao.cc> wrote:
> 
> > 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?
> 
> Is it possible to try executing one of the instructions and see if it traps?
> That saves having to maintain a list of broken cpu.

Sounds like a good idea, I'll give it a try with extable.

> 	David

Regards,
Yao Zi

> > 
> > > > +		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
> > 
>