target-i386: Fallback vcpu's TSC rate to value returned by KVM

If no user-specified TSC rate is present, we will try to set
env->tsc_khz to the value returned by KVM_GET_TSC_KHZ. This patch
does not change the current functionality of QEMU and just
prepares for later patches to enable migrating vcpu's TSC rate.
Signed-off-by: NHaozhong Zhang <haozhong.zhang@intel.com>
Reviewed-by: NEduardo Habkost <ehabkost@redhat.com>
Signed-off-by: NEduardo Habkost <ehabkost@redhat.com>

target-i386/kvm.c

@@ -868,6 +868,20 @@ int kvm_arch_init_vcpu(CPUState *cs)
         }
     }
 
+    /* vcpu's TSC frequency is either specified by user, or following
+     * the value used by KVM if the former is not present. In the
+     * latter case, we query it from KVM and record in env->tsc_khz,
+     * so that vcpu's TSC frequency can be migrated later via this field.
+     */
+    if (!env->tsc_khz) {
+        r = kvm_check_extension(cs->kvm_state, KVM_CAP_GET_TSC_KHZ) ?
+            kvm_vcpu_ioctl(cs, KVM_GET_TSC_KHZ) :
+            -ENOTSUP;
+        if (r > 0) {
+            env->tsc_khz = r;
+        }
+    }
+
     if (has_xsave) {
         env->kvm_xsave_buf = qemu_memalign(4096, sizeof(struct kvm_xsave));
     }