KVM: x86: disable kvmclock on non constant TSC hosts

[Marcelo Tosatti <mtosatti@redhat.com>]

Currently, this code path is posing us big troubles,
and we won't have a decent patch in time. So, temporarily
disable it.

There's a module parameter for the adventurous who want to force
it.

Signed-off-by: Glauber Costa <glommer@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
---
 arch/x86/kvm/x86.c |    7 ++++++-
 1 files changed, 6 insertions(+), 1 deletions(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index e96edda..6286f08 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -979,6 +979,9 @@ out:
 	return r;
 }
 
+static int force_kvmclock = 0;
+module_param(force_kvmclock, bool, 0644);
+
 int kvm_dev_ioctl_check_extension(long ext)
 {
 	int r;
@@ -989,7 +992,6 @@ int kvm_dev_ioctl_check_extension(long ext)
 	case KVM_CAP_MMU_SHADOW_CACHE_CONTROL:
 	case KVM_CAP_SET_TSS_ADDR:
 	case KVM_CAP_EXT_CPUID:
-	case KVM_CAP_CLOCKSOURCE:
 	case KVM_CAP_PIT:
 	case KVM_CAP_NOP_IO_DELAY:
 	case KVM_CAP_MP_STATE:
@@ -1015,6 +1017,9 @@ int kvm_dev_ioctl_check_extension(long ext)
 	case KVM_CAP_IOMMU:
 		r = iommu_found();
 		break;
+	case KVM_CAP_CLOCKSOURCE:
+		r = force_kvmclock || boot_cpu_has(X86_FEATURE_CONSTANT_TSC);
+		break;
 	default:
 		r = 0;
 		break;