[Qemu-devel] [PULL 06/20] i386/hyperv: add hv-frequencies cpu property

[Paolo Bonzini <pbonzini@redhat.com>]

From: Roman Kagan <rkagan@virtuozzo.com>

In order to guarantee compatibility on migration, QEMU should have
complete control over the features it announces to the guest via CPUID.

However, the availability of Hyper-V frequency MSRs
(HV_X64_MSR_TSC_FREQUENCY and HV_X64_MSR_APIC_FREQUENCY) depends solely
on the support for them in the underlying KVM.

Introduce "hv-frequencies" cpu property (off by default) which gives
QEMU full control over whether these MSRs are announced.

While at this, drop the redundant check of the cpu tsc frequency, and
decouple this feature from hv-time.

Signed-off-by: Roman Kagan <rkagan@virtuozzo.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20180330170209.20627-2-rkagan@virtuozzo.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
 target/i386/cpu.c |  1 +
 target/i386/cpu.h |  1 +
 target/i386/kvm.c | 13 +++++++++----
 3 files changed, 11 insertions(+), 4 deletions(-)

diff --git a/target/i386/cpu.c b/target/i386/cpu.c
index 555ae79..1a6b082 100644
--- a/target/i386/cpu.c
+++ b/target/i386/cpu.c
@@ -4761,6 +4761,7 @@ static Property x86_cpu_properties[] = {
     DEFINE_PROP_BOOL("hv-runtime", X86CPU, hyperv_runtime, false),
     DEFINE_PROP_BOOL("hv-synic", X86CPU, hyperv_synic, false),
     DEFINE_PROP_BOOL("hv-stimer", X86CPU, hyperv_stimer, false),
+    DEFINE_PROP_BOOL("hv-frequencies", X86CPU, hyperv_frequencies, false),
     DEFINE_PROP_BOOL("check", X86CPU, check_cpuid, true),
     DEFINE_PROP_BOOL("enforce", X86CPU, enforce_cpuid, false),
     DEFINE_PROP_BOOL("kvm", X86CPU, expose_kvm, true),
diff --git a/target/i386/cpu.h b/target/i386/cpu.h
index 78db1b8..1b219fa 100644
--- a/target/i386/cpu.h
+++ b/target/i386/cpu.h
@@ -1296,6 +1296,7 @@ struct X86CPU {
     bool hyperv_runtime;
     bool hyperv_synic;
     bool hyperv_stimer;
+    bool hyperv_frequencies;
     bool check_cpuid;
     bool enforce_cpuid;
     bool expose_kvm;
diff --git a/target/i386/kvm.c b/target/i386/kvm.c
index d23fff1..b35623a 100644
--- a/target/i386/kvm.c
+++ b/target/i386/kvm.c
@@ -648,11 +648,16 @@ static int hyperv_handle_properties(CPUState *cs)
         env->features[FEAT_HYPERV_EAX] |= HV_HYPERCALL_AVAILABLE;
         env->features[FEAT_HYPERV_EAX] |= HV_TIME_REF_COUNT_AVAILABLE;
         env->features[FEAT_HYPERV_EAX] |= HV_REFERENCE_TSC_AVAILABLE;
-
-        if (has_msr_hv_frequencies && tsc_is_stable_and_known(env)) {
-            env->features[FEAT_HYPERV_EAX] |= HV_ACCESS_FREQUENCY_MSRS;
-            env->features[FEAT_HYPERV_EDX] |= HV_FREQUENCY_MSRS_AVAILABLE;
+    }
+    if (cpu->hyperv_frequencies) {
+        if (!has_msr_hv_frequencies) {
+            fprintf(stderr, "Hyper-V frequency MSRs "
+                    "(requested by 'hv-frequencies' cpu flag) "
+                    "are not supported by kernel\n");
+            return -ENOSYS;
         }
+        env->features[FEAT_HYPERV_EAX] |= HV_ACCESS_FREQUENCY_MSRS;
+        env->features[FEAT_HYPERV_EDX] |= HV_FREQUENCY_MSRS_AVAILABLE;
     }
     if (cpu->hyperv_crash && has_msr_hv_crash) {
         env->features[FEAT_HYPERV_EDX] |= HV_GUEST_CRASH_MSR_AVAILABLE;
-- 
1.8.3.1