i386/cpu: Fix number of addressable IDs field for CPUID.01H.EBX[23:16]

When QEMU is started with:
-cpu host,migratable=on,host-cache-info=on,l3-cache=off
-smp 180,sockets=2,dies=1,cores=45,threads=2

On Intel platform:
CPUID.01H.EBX[23:16] is defined as "max number of addressable IDs for
logical processors in the physical package".

When executing "cpuid -1 -l 1 -r" in the guest, we obtain a value of 90 for
CPUID.01H.EBX[23:16], whereas the expected value is 128. Additionally,
executing "cpuid -1 -l 4 -r" in the guest yields a value of 63 for
CPUID.04H.EAX[31:26], which matches the expected result.

As (1+CPUID.04H.EAX[31:26]) rounds up to the nearest power-of-2 integer,
it's necessary to round up CPUID.01H.EBX[23:16] to the nearest power-of-2
integer too. Otherwise there would be unexpected results in guest with
older kernel.

For example, when QEMU is started with CLI above and xtopology is disabled,
guest kernel 5.15.120 uses CPUID.01H.EBX[23:16]/(1+CPUID.04H.EAX[31:26]) to
calculate threads-per-core in detect_ht(). Then guest will get "90/(1+63)=1"
as the result, even though threads-per-core should actually be 2.

And on AMD platform:
CPUID.01H.EBX[23:16] is defined as "Logical processor count". Current
result meets our expectation.

So round up CPUID.01H.EBX[23:16] to the nearest power-of-2 integer only
for Intel platform to solve the unexpected result.

Use the "x-vendor-cpuid-only-v2" compat option to fix this issue.

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Guixiong Wei <weiguixiong@bytedance.com>
Signed-off-by: Yipeng Yin <yinyipeng@bytedance.com>
Signed-off-by: Chuang Xu <xuchuangxclwt@bytedance.com>
Signed-off-by: Zhao Liu <zhao1.liu@intel.com>
Link: https://lore.kernel.org/r/20250714080859.1960104-5-zhao1.liu@intel.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 3b7c22e5d38f67708733f18a73ae52f5f1022836..12e719e99573917dcfd1a5e013588c3e5e759d6a 100644 (file)
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -7871,7 +7871,17 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
         }
         *edx = env->features[FEAT_1_EDX];
         if (threads_per_pkg > 1) {
-            *ebx |= threads_per_pkg << 16;
+            /*
+             * For CPUID.01H.EBX[Bits 23-16], AMD requires logical processor
+             * count, but Intel needs maximum number of addressable IDs for
+             * logical processors per package.
+             */
+            if (cpu->vendor_cpuid_only_v2 &&
+                (IS_INTEL_CPU(env) || IS_ZHAOXIN_CPU(env))) {
+                *ebx |= 1 << apicid_pkg_offset(topo_info) << 16;
+            } else {
+                *ebx |= threads_per_pkg << 16;
+            }
         }
         break;
     case 2: { /* cache info: needed for Pentium Pro compatibility */