[Qemu-devel] X86 CPU topology broken in qemu ?

[Bharata B Rao]

Hi,

I see that x86 CPU topology inside VM is not showing up as specified.
With some debugging, I found out that the root cause for this: qemu is
not enumerating the apic ids correctly for vcpus. I made the below
hackish change to get it working.  Has anybody else seen this problem
?  This patch is on qemu-kvm-0.14.1. Using 2.6.39 for guest.

***************************
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.

Signed-off-by: Bharata B Rao <address@hidden>
---
 hw/pc.c           |    4 +---
 target-i386/kvm.c |    3 +++
 2 files changed, 4 insertions(+), 3 deletions(-)

Index: qemu-kvm-0.14.1/hw/pc.c
===================================================================
--- qemu-kvm-0.14.1.orig/hw/pc.c
+++ qemu-kvm-0.14.1/hw/pc.c
@@ -930,10 +930,8 @@ CPUState *pc_new_cpu(const char *cpu_mod
        fprintf(stderr, "Unable to find x86 CPU definition\n");
        exit(1);
    }
-    if ((env->cpuid_features & CPUID_APIC) || smp_cpus > 1) {
-        env->cpuid_apic_id = env->cpu_index;
+    if ((env->cpuid_features & CPUID_APIC) || smp_cpus > 1)
        env->apic_state = apic_init(env, env->cpuid_apic_id);
-    }
    qemu_register_reset(pc_cpu_reset, env);
    pc_cpu_reset(env);
    return env;
Index: qemu-kvm-0.14.1/target-i386/kvm.c
===================================================================
--- qemu-kvm-0.14.1.orig/target-i386/kvm.c
+++ qemu-kvm-0.14.1/target-i386/kvm.c
@@ -340,6 +340,9 @@ int kvm_arch_init_vcpu(CPUState *env)

    cpuid_i = 0;

+    if (env->cpuid_features & CPUID_APIC)
+        env->cpuid_apic_id = env->cpu_index;
+
 #ifdef CONFIG_KVM_PARA
    /* Paravirtualization CPUIDs */
    memcpy(signature, "KVMKVMKVM\0\0\0", 12);

**************************

This is how various fields look like before and after this change with
qemu command line option of "-smp ,sockets=1,cores=4,threads=2"

Before
------
address@hidden:~# grep "core id" /proc/cpuinfo
core id         : 0
core id         : 0
core id         : 0
core id         : 0
core id         : 0
core id         : 0
core id         : 0
core id         : 0

After
-----
address@hidden:~# grep "core id" /proc/cpuinfo
core id         : 0
core id         : 0
core id         : 1
core id         : 1
core id         : 2
core id         : 2
core id         : 3
core id         : 3

Before
------
address@hidden:~# grep "cpu cores" /proc/cpuinfo
cpu cores       : 1
cpu cores       : 1
cpu cores       : 1
cpu cores       : 1
cpu cores       : 1
cpu cores       : 1
cpu cores       : 1
cpu cores       : 1

After
-----
address@hidden:~# grep "cpu cores" /proc/cpuinfo
cpu cores       : 4
cpu cores       : 4
cpu cores       : 4
cpu cores       : 4
cpu cores       : 4
cpu cores       : 4
cpu cores       : 4
cpu cores       : 4

Before
------
address@hidden:~# grep apicid /proc/cpuinfo
apicid          : 0
initial apicid  : 0
apicid          : 0
initial apicid  : 0
apicid          : 0
initial apicid  : 0
apicid          : 0
initial apicid  : 0
apicid          : 0
initial apicid  : 0
apicid          : 0
initial apicid  : 0
apicid          : 0
initial apicid  : 0
apicid          : 0
initial apicid  : 0

After
-----
address@hidden:~# grep apicid /proc/cpuinfo
apicid          : 0
initial apicid  : 0
apicid          : 1
initial apicid  : 1
apicid          : 2
initial apicid  : 2
apicid          : 3
initial apicid  : 3
apicid          : 4
initial apicid  : 4
apicid          : 5
initial apicid  : 5
apicid          : 6
initial apicid  : 6
apicid          : 7
initial apicid  : 7

Before
------
address@hidden:/sys/devices/system/cpu# cat cpu*/topology/core_siblings_list
0-7
0-7
0-7
0-7
0-7
0-7
0-7
0-7
address@hidden:/sys/devices/system/cpu# cat cpu*/topology/thread_siblings_list
0-7
0-7
0-7
0-7
0-7
0-7
0-7
0-7

After
-----
address@hidden:/sys/devices/system/cpu# cat cpu*/topology/core_siblings_list
0-7
0-7
0-7
0-7
0-7
0-7
0-7
0-7
address@hidden:/sys/devices/system/cpu# cat cpu*/topology/thread_siblings_list
0-1
0-1
2-3
2-3
4-5
4-5
6-7
6-7

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