[Qemu-devel] [RFC] Enabling x2apic on most (all?) x86 CPU models

[Eduardo Habkost]

Hi,

I would like to get your opinion on this:

Currently we have x2apic enabled only on SandyBridge and Haswell CPU
models because we try to keep the CPU models closer to real CPUs.
However, x2apic improves performance by reducing the overhead of APIC
accesses, and KVM can emulate it independently of host CPU support for
x2apic. This feature is present on KVM for 4 years, already (since
v2.6.32). There's no reason for people to not have x2apic enabled when
running KVM.

So, my question is: should we break the "try to be close to real CPUs"
rule and enable x2apic by default on most (or all) CPU models? I believe
it is a reasonable thing to do.

Also: if we do it, should we do it for all CPU models on
target-i386/cpu.c, or just a subset of them? (maybe the more recent
ones?)

(The patch below touches only Conroe, Penryn, Nehalem, and Westmere, and
it lacks machine-type compatibility code. But I am planning to submit a
patch that changes all CPU models to include x2apic by default.)

---
diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index 9abb73f..f76c34b 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -791,7 +791,7 @@ static x86_def_t builtin_x86_defs[] = {
              CPUID_MCE | CPUID_PAE | CPUID_MSR | CPUID_TSC | CPUID_PSE |
              CPUID_DE | CPUID_FP87,
         .features[FEAT_1_ECX] =
-            CPUID_EXT_SSSE3 | CPUID_EXT_SSE3,
+            CPUID_EXT_SSSE3 | CPUID_EXT_SSE3 | CPUID_EXT_X2APIC,
         .features[FEAT_8000_0001_EDX] =
             CPUID_EXT2_LM | CPUID_EXT2_NX | CPUID_EXT2_SYSCALL,
         .features[FEAT_8000_0001_ECX] =
@@ -814,7 +814,7 @@ static x86_def_t builtin_x86_defs[] = {
              CPUID_DE | CPUID_FP87,
         .features[FEAT_1_ECX] =
             CPUID_EXT_SSE41 | CPUID_EXT_CX16 | CPUID_EXT_SSSE3 |
-             CPUID_EXT_SSE3,
+             CPUID_EXT_SSE3 | CPUID_EXT_X2APIC,
         .features[FEAT_8000_0001_EDX] =
             CPUID_EXT2_LM | CPUID_EXT2_NX | CPUID_EXT2_SYSCALL,
         .features[FEAT_8000_0001_ECX] =
@@ -837,7 +837,8 @@ static x86_def_t builtin_x86_defs[] = {
              CPUID_DE | CPUID_FP87,
         .features[FEAT_1_ECX] =
             CPUID_EXT_POPCNT | CPUID_EXT_SSE42 | CPUID_EXT_SSE41 |
-             CPUID_EXT_CX16 | CPUID_EXT_SSSE3 | CPUID_EXT_SSE3,
+             CPUID_EXT_CX16 | CPUID_EXT_SSSE3 | CPUID_EXT_SSE3 |
+             CPUID_EXT_X2APIC,
         .features[FEAT_8000_0001_EDX] =
             CPUID_EXT2_LM | CPUID_EXT2_SYSCALL | CPUID_EXT2_NX,
         .features[FEAT_8000_0001_ECX] =
@@ -861,7 +862,7 @@ static x86_def_t builtin_x86_defs[] = {
         .features[FEAT_1_ECX] =
             CPUID_EXT_AES | CPUID_EXT_POPCNT | CPUID_EXT_SSE42 |
              CPUID_EXT_SSE41 | CPUID_EXT_CX16 | CPUID_EXT_SSSE3 |
-             CPUID_EXT_PCLMULQDQ | CPUID_EXT_SSE3,
+             CPUID_EXT_PCLMULQDQ | CPUID_EXT_SSE3 | CPUID_EXT_X2APIC,
         .features[FEAT_8000_0001_EDX] =
             CPUID_EXT2_LM | CPUID_EXT2_SYSCALL | CPUID_EXT2_NX,
         .features[FEAT_8000_0001_ECX] =