[Qemu-devel] [qom-cpu PATCH 2/2] i386: disable PMU passthrough mode by default

[Eduardo Habkost <ehabkost@redhat.com>]

Bug description: QEMU currently gets all bits from GET_SUPPORTED_CPUID
for CPUID leaf 0xA and passes them directly to the guest. This makes
the guest ABI depend on host kernel and host CPU capabilities, and
breaks live migration if we migrate between host with different
capabilities (e.g. different number of PMU counters).

This patch adds a "pmu-passthrough" property to X86CPU, and set it to
true only on "-cpu host", or on pc-*-1.5 and older machine-types.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 include/hw/i386/pc.h  |  4 ++++
 target-i386/cpu-qom.h |  7 +++++++
 target-i386/cpu.c     | 11 ++++++++++-
 3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
index 7fb97b0..3cea83f 100644
--- a/include/hw/i386/pc.h
+++ b/include/hw/i386/pc.h
@@ -235,6 +235,10 @@ int e820_add_entry(uint64_t, uint64_t, uint32_t);
             .driver   = "virtio-net-pci",\
             .property = "any_layout",\
             .value    = "off",\
+        },{\
+            .driver = TYPE_X86_CPU,\
+            .property = "pmu-passthrough",\
+            .value = "on",\
         }
 
 #define PC_COMPAT_1_4 \
diff --git a/target-i386/cpu-qom.h b/target-i386/cpu-qom.h
index 7e55e5f..b505a45 100644
--- a/target-i386/cpu-qom.h
+++ b/target-i386/cpu-qom.h
@@ -68,6 +68,13 @@ typedef struct X86CPU {
 
     /* Features that were filtered out because of missing host capabilities */
     uint32_t filtered_features[FEATURE_WORDS];
+
+    /* Pass all PMU CPUID bits to the guest directly from GET_SUPPORTED_CPUID.
+     * This can't be enabled by default because it breaks live-migration,
+     * as it makes the guest ABI change depending on host CPU/kernel
+     * capabilities.
+     */
+    bool pmu_passthrough;
 } X86CPU;
 
 static inline X86CPU *x86_env_get_cpu(CPUX86State *env)
diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index 41c81af..e192f63 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -1475,17 +1475,25 @@ static void x86_cpu_get_feature_words(Object *obj, Visitor *v, void *opaque,
     error_propagate(errp, err);
 }
 
+static Property cpu_x86_properties[] = {
+    DEFINE_PROP_BOOL("pmu-passthrough", X86CPU, pmu_passthrough, false),
+    DEFINE_PROP_END_OF_LIST(),
+};
+
 static int cpu_x86_find_by_name(X86CPU *cpu, x86_def_t *x86_cpu_def,
                                 const char *name)
 {
     x86_def_t *def;
     int i;
+    Error *err = NULL;
 
     if (name == NULL) {
         return -1;
     }
     if (kvm_enabled() && strcmp(name, "host") == 0) {
         kvm_cpu_fill_host(x86_cpu_def);
+        object_property_set_bool(OBJECT(cpu), true, "pmu-passthrough", &err);
+        assert_no_error(err);
         return 0;
     }
 
@@ -2017,7 +2025,7 @@ void cpu_x86_cpuid(CPUX86State *env, uint32_t index, uint32_t count,
         break;
     case 0xA:
         /* Architectural Performance Monitoring Leaf */
-        if (kvm_enabled()) {
+        if (kvm_enabled() && cpu->pmu_passthrough) {
             KVMState *s = cs->kvm_state;
 
             *eax = kvm_arch_get_supported_cpuid(s, 0xA, count, R_EAX);
@@ -2516,6 +2524,7 @@ static void x86_cpu_common_class_init(ObjectClass *oc, void *data)
     xcc->parent_realize = dc->realize;
     dc->realize = x86_cpu_realizefn;
     dc->bus_type = TYPE_ICC_BUS;
+    dc->props = cpu_x86_properties;
 
     xcc->parent_reset = cc->reset;
     cc->reset = x86_cpu_reset;
-- 
1.8.1.4