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[Qemu-devel] [FIX] X86 CPU topology broken in KVM mode

From: Bharata B Rao
Subject: [Qemu-devel] [FIX] X86 CPU topology broken in KVM mode
Date: Wed, 7 Sep 2011 09:51:41 +0530


Sometime back I posted a patch for fixing x86 CPU topology (
Here is the next version of the fix which addresses all but one
comment received during that post.

- Fixed code style issues
- Ensured that the fix doesn't break TCG mode
- I am not sure what is the problem with i486 as I haven't been able
to boot an i486 VM successfully, hence haven't attempted to fix this.

I have tested following scenarios and found the fix to be working fine.

KVM: (with --enable-kvm)
-smp sockets=1,cores=4,threads=2
-smp sockets=4,cores=4,threads=2
-cpu core2duo sockets=1,cores=4,threads=2
-cpu core2duo sockets=2,cores=4,threads=2

TCG: (without --enable-kvm)
-cpu core2duo sockets=1,cores=4,threads=2
-cpu core2duo sockets=2,cores=4,threads=2

Here is the updated patch which now applies against qemu.git.

Fix apic id enumeration

apic id returned to guest kernel in ebx for cpuid(function=1) depends on
CPUX86State->cpuid_apic_id which gets populated after the cpuid information
is cached in the host kernel.

Fix this by setting cpuid_apic_id before cpuid information is passed to
the host kernel. This is done by moving the setting of cpuid_apic_id
to cpu_x86_init() where it will work for both KVM as well as TCG modes.

Signed-off-by: Bharata B Rao <address@hidden>
 hw/pc.c              |    1 -
 target-i386/helper.c |    5 +++++
 2 files changed, 5 insertions(+), 1 deletion(-)

Index: qemu/hw/pc.c
--- qemu.orig/hw/pc.c
+++ qemu/hw/pc.c
@@ -933,7 +933,6 @@ static CPUState *pc_new_cpu(const char *
     if ((env->cpuid_features & CPUID_APIC) || smp_cpus > 1) {
-        env->cpuid_apic_id = env->cpu_index;
         env->apic_state = apic_init(env, env->cpuid_apic_id);
     qemu_register_reset(pc_cpu_reset, env);
Index: qemu/target-i386/helper.c
--- qemu.orig/target-i386/helper.c
+++ qemu/target-i386/helper.c
@@ -1256,6 +1256,11 @@ CPUX86State *cpu_x86_init(const char *cp
         return NULL;
+    if (env->cpuid_features & CPUID_APIC) {
+        env->cpuid_apic_id = env->cpu_index;
+    }


 http://bharata.sulekha.com/blog/posts.htm, http://raobharata.wordpress.com/

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