[Qemu-devel] [PATCH] target-i386: Support migratable=no properly

[Eduardo Habkost <ehabkost@redhat.com>]

When the "migratable" property was implemented, the behavior was tested
by changing the default on the code, but actually using the option on
the command-line (e.g. "-cpu host,migratable=false") doesn't work as
expected. This is a regression for a common use case of "-cpu host",
which is to enable features that are supported by the host CPU + kernel
before feature-specific code is added to QEMU.

Fix this by initializing the feature words for "-cpu host" on
x86_cpu_parse_featurestr(), right after parsing the CPU options.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
I was considering a more complex (but less hacky) fix, by introducing an
enum FeatureBit { OFF, ON, HOST }, making FeatureWord a 32-element
FeatureBit array, and initializing the actual feature bits on
x86_cpu_realizefn(). But as this is a patch for qemu-stable, I kept the
fix as simple as possible.
---
 target-i386/cpu-qom.h |  1 +
 target-i386/cpu.c     | 17 ++++++++++++-----
 2 files changed, 13 insertions(+), 5 deletions(-)

diff --git a/target-i386/cpu-qom.h b/target-i386/cpu-qom.h
index 71a1b97..7755466 100644
--- a/target-i386/cpu-qom.h
+++ b/target-i386/cpu-qom.h
@@ -92,6 +92,7 @@ typedef struct X86CPU {
     bool enforce_cpuid;
     bool expose_kvm;
     bool migratable;
+    bool host_features;
 
     /* if true the CPUID code directly forward host cache leaves to the guest */
     bool cache_info_passthrough;
diff --git a/target-i386/cpu.c b/target-i386/cpu.c
index 6d008ab..c0f8efc 100644
--- a/target-i386/cpu.c
+++ b/target-i386/cpu.c
@@ -1318,18 +1318,18 @@ static void host_x86_cpu_initfn(Object *obj)
     X86CPU *cpu = X86_CPU(obj);
     CPUX86State *env = &cpu->env;
     KVMState *s = kvm_state;
-    FeatureWord w;
 
     assert(kvm_enabled());
 
+    /* We can't fill the features array here because we don't know yet if
+     * "migratable" is true or false.
+     */
+    cpu->host_features = true;
+
     env->cpuid_level = kvm_arch_get_supported_cpuid(s, 0x0, 0, R_EAX);
     env->cpuid_xlevel = kvm_arch_get_supported_cpuid(s, 0x80000000, 0, R_EAX);
     env->cpuid_xlevel2 = kvm_arch_get_supported_cpuid(s, 0xC0000000, 0, R_EAX);
 
-    for (w = 0; w < FEATURE_WORDS; w++) {
-        env->features[w] =
-            x86_cpu_get_supported_feature_word(w, cpu->migratable);
-    }
     object_property_set_bool(OBJECT(cpu), true, "pmu", &error_abort);
 }
 
@@ -1828,6 +1828,13 @@ static void x86_cpu_parse_featurestr(CPUState *cs, char *features,
         featurestr = strtok(NULL, ",");
     }
 
+    if (cpu->host_features) {
+        for (w = 0; w < FEATURE_WORDS; w++) {
+            env->features[w] =
+                x86_cpu_get_supported_feature_word(w, cpu->migratable);
+        }
+    }
+
     for (w = 0; w < FEATURE_WORDS; w++) {
         env->features[w] |= plus_features[w];
         env->features[w] &= ~minus_features[w];
-- 
1.9.3