allow CPUID vendor override

KVM-enabled QEMU will always report the vendor ID of the physical CPU it is
running on. Allow to override this if explicitly requested on the
command line. It will not suffice to name a CPU type (like -cpu phenom),
but you have to explicitly set the vendor: -cpu phenom,vendor=AuthenticAMD
Signed-off-by: NAndre Przywara <andre.przywara@amd.com>
Signed-off-by: NAnthony Liguori <aliguori@us.ibm.com>

target-i386/cpu.h

@@ -655,6 +655,7 @@ typedef struct CPUX86State {
     uint32_t cpuid_ext2_features;
     uint32_t cpuid_ext3_features;
     uint32_t cpuid_apic_id;
+    int cpuid_vendor_override;
 
     /* MTRRs */
     uint64_t mtrr_fixed[11];

target-i386/helper.c

@@ -116,6 +116,7 @@ typedef struct x86_def_t {
     uint32_t features, ext_features, ext2_features, ext3_features;
     uint32_t xlevel;
     char model_id[48];
+    int vendor_override;
 } x86_def_t;
 
 #define I486_FEATURES (CPUID_FP87 | CPUID_VME | CPUID_PSE)
@@ -376,6 +377,7 @@ static int cpu_x86_find_by_name(x86_def_t *x86_cpu_def, const char *cpu_model)
                     x86_cpu_def->vendor2 |= ((uint8_t)val[i + 4]) << (8 * i);
                     x86_cpu_def->vendor3 |= ((uint8_t)val[i + 8]) << (8 * i);
                 }
+                x86_cpu_def->vendor_override = 1;
             } else if (!strcmp(featurestr, "model_id")) {
                 pstrcpy(x86_cpu_def->model_id, sizeof(x86_cpu_def->model_id),
                         val);
@@ -428,6 +430,7 @@ static int cpu_x86_register (CPUX86State *env, const char *cpu_model)
         env->cpuid_vendor2 = CPUID_VENDOR_INTEL_2;
         env->cpuid_vendor3 = CPUID_VENDOR_INTEL_3;
     }
+    env->cpuid_vendor_override = def->vendor_override;
     env->cpuid_level = def->level;
     if (def->family > 0x0f)
         env->cpuid_version = 0xf00 | ((def->family - 0x0f) << 20);
@@ -1506,7 +1509,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
          * isn't supported in compatibility mode on Intel.  so advertise the
          * actuall cpu, and say goodbye to migration between different vendors
          * is you use compatibility mode. */
-        if (kvm_enabled())
+        if (kvm_enabled() && !env->cpuid_vendor_override)
             host_cpuid(0, 0, NULL, ebx, ecx, edx);
         break;
     case 1: