target/i386: Mask CMPLegacy bit in CPUID[0x80000001].ECX for Zhaoxin CPUs

Zhaoxin CPUs (including vendors "Shanghai" and "Centaurhauls") handle the
CMPLegacy bit similarly to Intel CPUs. Therefore, this commit masks the
CMPLegacy bit in CPUID[0x80000001].ECX for Zhaoxin CPUs, just as it is done
for Intel CPUs.

AMD uses the CMPLegacy bit (CPUID[0x80000001].ECX.bit1) along with other CPUID
information to enumerate platform topology (e.g., the number of logical
processors per package). However, for Intel and other CPUs that follow Intel's
behavior, CPUID[0x80000001].ECX.bit1 is reserved.

- Impact on Intel and similar CPUs:
This change has no effect on Intel and similar CPUs, as the goal is to
accurately emulate CPU CPUID information.

- Impact on Linux Guests running on Intel (and similar) vCPUs:
During boot, Linux checks if the CPU supports Hyper-Threading. For the Linux
kernel before v6.9, if it detects X86_FEATURE_CMP_LEGACY, it assumes
Hyper-Threading is not supported. For Intel and similar vCPUs, if the
CMPLegacy bit is not masked in CPUID[0x80000001].ECX, Linux will incorrectly
assume that Hyper-Threading is not supported, even if the vCPU does support it.

Signed-off-by: EwanHai <ewanhai-oc@zhaoxin.com>
Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250113074413.297793-5-ewanhai-oc@zhaoxin.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 3 insertions, 2 deletions

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index bd40714..0cd9b70 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -7804,9 +7804,10 @@ void x86_cpu_expand_features(X86CPU *cpu, Error **errp)
         /*
          * The Linux kernel checks for the CMPLegacy bit and
          * discards multiple thread information if it is set.
-         * So don't set it here for Intel to make Linux guests happy.
+         * So don't set it here for Intel (and other processors
+         * following Intel's behavior) to make Linux guests happy.
          */
-        if (!IS_INTEL_CPU(env)) {
+        if (!IS_INTEL_CPU(env) && !IS_ZHAOXIN_CPU(env)) {
             env->features[FEAT_8000_0001_ECX] |= CPUID_EXT3_CMP_LEG;
         }
     }