[Qemu-devel] [PATCH 4/4] kvm: Allow migration with invtsc

[Eduardo Habkost <ehabkost@redhat.com>]

Instead of blocking migration on the source when invtsc is
enabled, rely on the migration destination to ensure there's no
TSC frequency mismatch.

We can't allow migration unconditionally because we don't know if
the destination is a QEMU version that is really going to ensure
there's no TSC frequency mismatch. To ensure we are migrating to
a destination that won't ignore SET_TSC_KHZ errors, allow invtsc
migration only on pc-*-2.9 and newer.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
---
 include/hw/i386/pc.h |  7 ++++++-
 target/i386/cpu.h    |  1 +
 target/i386/cpu.c    |  1 +
 target/i386/kvm.c    | 15 +++++++++------
 4 files changed, 17 insertions(+), 7 deletions(-)

diff --git a/include/hw/i386/pc.h b/include/hw/i386/pc.h
index ceeacca..4270923 100644
--- a/include/hw/i386/pc.h
+++ b/include/hw/i386/pc.h
@@ -375,7 +375,12 @@ int e820_get_num_entries(void);
 bool e820_get_entry(int, uint32_t, uint64_t *, uint64_t *);
 
 #define PC_COMPAT_2_8 \
-    HW_COMPAT_2_8
+    HW_COMPAT_2_8 \
+    {\
+        .driver   = TYPE_X86_CPU,\
+        .property = "invtsc-migration",\
+        .value    = "off",\
+    },
 
 #define PC_COMPAT_2_7 \
     HW_COMPAT_2_7 \
diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index a7f2f60..ec8cdbc 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -1208,6 +1208,7 @@ struct X86CPU {
     bool expose_kvm;
     bool migratable;
     bool host_features;
+    bool invtsc_migration;
     uint32_t apic_id;
 
     /* if true the CPUID code directly forward host cache leaves to the guest */
diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index b0640f1..cc93b81 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -3678,6 +3678,7 @@ static Property x86_cpu_properties[] = {
     DEFINE_PROP_BOOL("cpuid-0xb", X86CPU, enable_cpuid_0xb, true),
     DEFINE_PROP_BOOL("lmce", X86CPU, enable_lmce, false),
     DEFINE_PROP_BOOL("l3-cache", X86CPU, enable_l3_cache, true),
+    DEFINE_PROP_BOOL("invtsc-migration", X86CPU, invtsc_migration, true),
     DEFINE_PROP_END_OF_LIST()
 };
 
diff --git a/target/i386/kvm.c b/target/i386/kvm.c
index 6a51399..2c3ee7b 100644
--- a/target/i386/kvm.c
+++ b/target/i386/kvm.c
@@ -962,7 +962,7 @@ int kvm_arch_init_vcpu(CPUState *cs)
         has_msr_mcg_ext_ctl = has_msr_feature_control = true;
     }
 
-    if (!env->user_tsc_khz) {
+    if (!cpu->invtsc_migration && !env->user_tsc_khz) {
         if ((env->features[FEAT_8000_0007_EDX] & CPUID_APM_INVTSC) &&
             invtsc_mig_blocker == NULL) {
             /* for migration */
@@ -972,6 +972,7 @@ int kvm_arch_init_vcpu(CPUState *cs)
             migrate_add_blocker(invtsc_mig_blocker);
             /* for savevm */
             vmstate_x86_cpu.unmigratable = 1;
+        }
     }
 
     cpuid_data.cpuid.padding = 0;
@@ -2655,12 +2656,14 @@ int kvm_arch_put_registers(CPUState *cpu, int level)
     }
 
     if (level == KVM_PUT_FULL_STATE) {
-        /* We don't check for kvm_arch_set_tsc_khz() errors here,
-         * because TSC frequency mismatch shouldn't abort migration,
-         * unless the user explicitly asked for a more strict TSC
-         * setting (e.g. using an explicit "tsc-freq" option).
+        /* Migration TSC frequency mismatch is fatal only if we are
+         * actually reporting Invariant TSC to the guest.
          */
-        kvm_arch_set_tsc_khz(cpu);
+        ret = kvm_arch_set_tsc_khz(cpu);
+        if ((x86_cpu->env.features[FEAT_8000_0007_EDX] & CPUID_APM_INVTSC) &&
+            ret < 0) {
+            return ret;
+        }
     }
 
     ret = kvm_getput_regs(x86_cpu, 1);
-- 
2.7.4