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

[Tony W Wang-oc]

Thank you for submitting the patch to fix the Zhaoxin CPU issue.

After internal clarification, we have confirmed that this is an
issue with the ZX-C CPU ucode:
When modifying CR4.FSGSBASE bit 16, the ucode propagates its
value to another MSR register. During execution of FSGSBASE-related
instructions, the hardware actually checks whether this MSR
register's bit is set to determine whether to generate a #UD
exception.
When the CPU enters SMM mode and then returns via RSM, the CR4
register is restored but the value of CR4.FSGSBASE is not
re-propagated to the MSR register.
As a result, after enabling CR4.FSGSBASE, once the CPU goes
through SMM mode, executing FSGSBASE-related instructions will
trigger a #UD exception.

This issue exists only on ZX-C CPUs, which have two different
CPU vendor IDs and distinct FMS values. The following patch can
be used to identify ZX-C CPUs and properly handle this issue:

--- a/arch/x86/kernel/cpu/centaur.c
+++ b/arch/x86/kernel/cpu/centaur.c
@@ -201,6 +201,11 @@ static void init_centaur(struct cpuinfo_x86 *c)
        set_cpu_cap(c, X86_FEATURE_LFENCE_RDTSC);
 #endif

+       if (c->x86 == 6 && c->x86_model == 15 && c->x86_stepping >= 14) {
+               pr_warn_once("CPU has broken FSGSBASE support; clear 
FSGSBASE feature\n");
+               setup_clear_cpu_cap(X86_FEATURE_FSGSBASE);
+       }
+
        init_ia32_feat_ctl(c);
 }

diff --git a/arch/x86/kernel/cpu/zhaoxin.c b/arch/x86/kernel/cpu/zhaoxin.c
index 031379b7d4fa..6a2d6df307ee 100644
--- a/arch/x86/kernel/cpu/zhaoxin.c
+++ b/arch/x86/kernel/cpu/zhaoxin.c
@@ -89,6 +89,11 @@ static void init_zhaoxin(struct cpuinfo_x86 *c)
        set_cpu_cap(c, X86_FEATURE_LFENCE_RDTSC);
 #endif

+       if (c->x86 == 6 && c->x86_model == 25 && c->x86_stepping <= 3) {
+               pr_warn_once("CPU has broken FSGSBASE support; clear 
FSGSBASE feature\n");
+               setup_clear_cpu_cap(X86_FEATURE_FSGSBASE);
+       }
+
        init_ia32_feat_ctl(c);
 }

Sincerely
TonyWWang-oc