[Qemu-devel] [PATCH v2 2/2] kvm: Allow invtsc migration if tsc-khz is set explicitly

[Eduardo Habkost <ehabkost@redhat.com>]

We can safely allow a VM to be migrated with invtsc enabled if
tsc-khz is set explicitly, because:
* QEMU already refuses to start if it can't set the TSC frequency
  to the configured value.
* Management software is already required to keep device
  configuration (including CPU configuration) the same on
  migration source and destination.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
Changes v1 -> v2:
* Reworded commit message
---
 target/i386/kvm.c | 19 ++++++++++---------
 1 file changed, 10 insertions(+), 9 deletions(-)

diff --git a/target/i386/kvm.c b/target/i386/kvm.c
index a26290fdc1..6a51399945 100644
--- a/target/i386/kvm.c
+++ b/target/i386/kvm.c
@@ -962,15 +962,16 @@ int kvm_arch_init_vcpu(CPUState *cs)
         has_msr_mcg_ext_ctl = has_msr_feature_control = true;
     }
 
-    if ((env->features[FEAT_8000_0007_EDX] & CPUID_APM_INVTSC) &&
-        invtsc_mig_blocker == NULL) {
-        /* for migration */
-        error_setg(&invtsc_mig_blocker,
-                   "State blocked by non-migratable CPU device"
-                   " (invtsc flag)");
-        migrate_add_blocker(invtsc_mig_blocker);
-        /* for savevm */
-        vmstate_x86_cpu.unmigratable = 1;
+    if (!env->user_tsc_khz) {
+        if ((env->features[FEAT_8000_0007_EDX] & CPUID_APM_INVTSC) &&
+            invtsc_mig_blocker == NULL) {
+            /* for migration */
+            error_setg(&invtsc_mig_blocker,
+                       "State blocked by non-migratable CPU device"
+                       " (invtsc flag)");
+            migrate_add_blocker(invtsc_mig_blocker);
+            /* for savevm */
+            vmstate_x86_cpu.unmigratable = 1;
     }
 
     cpuid_data.cpuid.padding = 0;
-- 
2.11.0.259.g40922b1