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

[Yao Zi <me@ziyao.cc>]

On Sun, Mar 01, 2026 at 04:29:13PM +0000, Andrew Cooper wrote:
> On 28/02/2026 5:37 pm, 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().
> 
> #UD is the expected behaviour of the FSGS instructions if they're not
> enabled.
> 
> Are you saying that this specific CPU enumerates FSGSBASE in CPUID, and
> permits setting CR4.FSGSBASE (without #GP for a reserved bit), and the
> FSGS instructions still do not function?

Yes. Without any workarounds, the kernel crashes in current_save_fsgs(),
which is the first use site of rdfsbase, instead of identify_cpu() where
CR4.FSGSBASE is set up.

> What happens if you read CR4 back after trying to set the bit?

CR4.FSGSBASE is set correctly, I wrote a small patch for testing,

diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 1c3261cae40c..d89a2cc71147 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -2048,8 +2048,13 @@ static void identify_cpu(struct cpuinfo_x86 *c)
 	setup_lass(c);
 
 	/* Enable FSGSBASE instructions if available. */
-	if (cpu_has(c, X86_FEATURE_FSGSBASE)) {
+	if (1) {
+		pr_info("%s: enabling FSGSBASE\n", __func__);
+		pr_info("%s: before enabling, CR4 = 0x%lx\n",
+			__func__, native_read_cr4());
 		cr4_set_bits(X86_CR4_FSGSBASE);
+		pr_info("%s: after enabling, CR4 = 0x%lx\n",
+			__func__, native_read_cr4());
 		elf_hwcap2 |= HWCAP2_FSGSBASE;
 	}

On BSP I got,

[    0.298016] identify_cpu: enabling FSGSBASE
[    0.298021] identify_cpu: before enabling, CR4 = 0x1200b0
[    0.298027] identify_cpu: after enabling, CR4 = 0x1300b0

and on APs, CR4.FSGSBASE seems to be set by default,

[    0.414981] smp: Bringing up secondary CPUs ...
[    0.415211] smpboot: x86: Booting SMP configuration:
[    0.415219] .... node  #0, CPUs:      #1 #2 #3
[    0.001869] identify_cpu: enabling FSGSBASE
[    0.001869] identify_cpu: before enabling, CR4 = 0x1706b0
[    0.001869] identify_cpu: after enabling, CR4 = 0x1706b0

> ~Andrew

Regards,
Yao Zi