[PATCH v1] target/i386: kvm: Block migration when enfore_cpuid is set to false

[Wei Wang]

When enforce_cpuid is set to false, the guest is launched with a filtered
set of features, meaning that unsupported features by the host are removed
from the guest's vCPU model. This could cause issues for live migration.
For example, a guest on the source is running with features A and B. If
the destination host does not support feature B, the stub guest can still
be launched on the destination with feature A only if enforce_cpuid=false.
Live migration can start in this case, though it may fail later when the
states of feature B are put to the destination side. This failure occurs
in the late stage (i.e., stop&copy phase) of the migration flow, where the
source guest has already been paused. Tests show that in such cases the
source guest does not recover, and the destination is unable to resume to
run.

Make "enfore_cpuid=true" a hard requirement for a guest to be migratable,
and change the default value of "enforce_cpuid" to true, making the guest
vCPUs migratable by default. If the destination stub guest has inconsistent
CPUIDs (i.e., destination host cannot support the features defined by the
guest's vCPU model), it fails to boot (with enfore_cpuid=true by default),
thereby preventing migration from occuring. If enfore_cpuid=false is
explicitly added for the guest, the guest is deemed as non-migratable
(via the migration blocker), so the above issue won't occur as the guest
won't be migrated.

Tested-by: Lei Wang <lei4.wang@intel.com>
Signed-off-by: Wei Wang <wei.w.wang@intel.com>
---
 target/i386/cpu.c     |  2 +-
 target/i386/kvm/kvm.c | 25 +++++++++++++++----------
 2 files changed, 16 insertions(+), 11 deletions(-)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 4c2e6f3a71..7db4fe4ead 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -8258,7 +8258,7 @@ static Property x86_cpu_properties[] = {
     DEFINE_PROP_UINT32("hv-version-id-snumber", X86CPU, hyperv_ver_id_sn, 0),
 
     DEFINE_PROP_BOOL("check", X86CPU, check_cpuid, true),
-    DEFINE_PROP_BOOL("enforce", X86CPU, enforce_cpuid, false),
+    DEFINE_PROP_BOOL("enforce", X86CPU, enforce_cpuid, true),
     DEFINE_PROP_BOOL("x-force-features", X86CPU, force_features, false),
     DEFINE_PROP_BOOL("kvm", X86CPU, expose_kvm, true),
     DEFINE_PROP_UINT32("phys-bits", X86CPU, phys_bits, 0),
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index dd8b0f3313..aee717c1cf 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -1741,7 +1741,7 @@ static int hyperv_init_vcpu(X86CPU *cpu)
     return 0;
 }
 
-static Error *invtsc_mig_blocker;
+static Error *cpu_mig_blocker;
 
 #define KVM_MAX_CPUID_ENTRIES  100
 
@@ -2012,6 +2012,15 @@ full:
     abort();
 }
 
+static bool kvm_vcpu_need_block_migration(X86CPU *cpu)
+{
+    CPUX86State *env = &cpu->env;
+
+    return !cpu->enforce_cpuid ||
+           (!env->user_tsc_khz && (env->features[FEAT_8000_0007_EDX] &
+                                   CPUID_APM_INVTSC));
+}
+
 int kvm_arch_init_vcpu(CPUState *cs)
 {
     struct {
@@ -2248,18 +2257,14 @@ int kvm_arch_init_vcpu(CPUState *cs)
         has_msr_mcg_ext_ctl = has_msr_feature_control = true;
     }
 
-    if (!env->user_tsc_khz) {
-        if ((env->features[FEAT_8000_0007_EDX] & CPUID_APM_INVTSC) &&
-            invtsc_mig_blocker == NULL) {
-            error_setg(&invtsc_mig_blocker,
-                       "State blocked by non-migratable CPU device"
-                       " (invtsc flag)");
-            r = migrate_add_blocker(&invtsc_mig_blocker, &local_err);
+    if (!cpu_mig_blocker &&  kvm_vcpu_need_block_migration(cpu)) {
+            error_setg(&cpu_mig_blocker,
+                       "State blocked by non-migratable CPU device");
+            r = migrate_add_blocker(&cpu_mig_blocker, &local_err);
             if (r < 0) {
                 error_report_err(local_err);
                 return r;
             }
-        }
     }
 
     if (cpu->vmware_cpuid_freq
@@ -2312,7 +2317,7 @@ int kvm_arch_init_vcpu(CPUState *cs)
     return 0;
 
  fail:
-    migrate_del_blocker(&invtsc_mig_blocker);
+    migrate_del_blocker(&cpu_mig_blocker);
 
     return r;
 }
-- 
2.27.0