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[v4,4/4] target/i386: Mask CMPLegacy bit in CPUID[0x80000001].ECX for Zhaoxin CPUs

Message ID 20240812101434.3804790-5-ewanhai-oc@zhaoxin.com (mailing list archive)
State New, archived
Headers show
Series Add support for Zhaoxin Yongfeng CPU model and | expand

Commit Message

EwanHai Aug. 12, 2024, 10:14 a.m. UTC 
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>
---
 target/i386/cpu.c | 7 +++----
 1 file changed, 3 insertions(+), 4 deletions(-)
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Patch

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 95849c40ad..eb55d92e8a 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -6995,12 +6995,11 @@  void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
 
         /* 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 (threads_per_pkg > 1) {
-            if (env->cpuid_vendor1 != CPUID_VENDOR_INTEL_1 ||
-                env->cpuid_vendor2 != CPUID_VENDOR_INTEL_2 ||
-                env->cpuid_vendor3 != CPUID_VENDOR_INTEL_3) {
+            if (!IS_INTEL_CPU(env) && !IS_ZHAOXIN_CPU(env)) {
                 *ecx |= 1 << 1;    /* CmpLegacy bit */
             }
         }