[PATCH] KVM: Don't automatically expose the TSC deadline timer in cpuid

[Avi Kivity <avi@redhat.com>]

From: Jan Kiszka <jan.kiszka@siemens.com>

Unlike all of the other cpuid bits, the TSC deadline timer bit is set
unconditionally, regardless of what userspace wants.

This is broken in several ways:
 - if userspace doesn't use KVM_CREATE_IRQCHIP, and doesn't emulate the TSC
   deadline timer feature, a guest that uses the feature will break
 - live migration to older host kernels that don't support the TSC deadline
   timer will cause the feature to be pulled from under the guest's feet;
   breaking it
 - guests that are broken wrt the feature will fail.

Fix by not enabling the feature automatically; instead report it to userspace.
Because the feature depends on KVM_CREATE_IRQCHIP, which we cannot guarantee
will be called, we expose it via a KVM_CAP_TSC_DEADLINE_TIMER and not
KVM_GET_SUPPORTED_CPUID.

Fixes the Illumos guest kernel, which uses the TSC deadline timer feature.

[avi: add the KVM_CAP + documentation]

Reported-by: Alexey Zaytsev <alexey.zaytsev@gmail.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
---

As we're running out of time and everyone's checking their socks instead of
inboxes I've added the missing parts myself.  Jan, if you accidentally see
this, please review and add your signoff.

 Documentation/virtual/kvm/api.txt |    9 +++++++++
 arch/x86/kvm/cpuid.c              |   16 ++++++----------
 arch/x86/kvm/x86.c                |    3 +++
 include/linux/kvm.h               |    1 +
 4 files changed, 19 insertions(+), 10 deletions(-)

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index 5b03eee..da1f8fd 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -1100,6 +1100,15 @@ emulate them efficiently. The fields in each entry are defined as follows:
    eax, ebx, ecx, edx: the values returned by the cpuid instruction for
          this function/index combination
 
+The TSC deadline timer feature (CPUID leaf 1, ecx[24]) is always returned
+as false, since the feature depends on KVM_CREATE_IRQCHIP for local APIC
+support.  Instead it is reported via
+
+  ioctl(KVM_CHECK_EXTENSION, KVM_CAP_TSC_DEADLINE_TIMER)
+
+if that returns true you use KVM_CREATE_IRQCHIP, or if emulate the
+feature in userspace, then you can enable the feature for KVM_SET_CPUID2.
+
 4.47 KVM_PPC_GET_PVINFO
 
 Capability: KVM_CAP_PPC_GET_PVINFO
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 230f713..89b02bf 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -27,7 +27,6 @@ void kvm_update_cpuid(struct kvm_vcpu *vcpu)
 {
 	struct kvm_cpuid_entry2 *best;
 	struct kvm_lapic *apic = vcpu->arch.apic;
-	u32 timer_mode_mask;
 
 	best = kvm_find_cpuid_entry(vcpu, 1, 0);
 	if (!best)
@@ -40,15 +39,12 @@ void kvm_update_cpuid(struct kvm_vcpu *vcpu)
 			best->ecx |= bit(X86_FEATURE_OSXSAVE);
 	}
 
-	if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL &&
-		best->function == 0x1) {
-		best->ecx |= bit(X86_FEATURE_TSC_DEADLINE_TIMER);
-		timer_mode_mask = 3 << 17;
-	} else
-		timer_mode_mask = 1 << 17;
-
-	if (apic)
-		apic->lapic_timer.timer_mode_mask = timer_mode_mask;
+	if (apic) {
+		if (best->ecx & bit(X86_FEATURE_TSC_DEADLINE_TIMER))
+			apic->lapic_timer.timer_mode_mask = 3 << 17;
+		else
+			apic->lapic_timer.timer_mode_mask = 1 << 17;
+	}
 
 	kvm_pmu_cpuid_update(vcpu);
 }
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index df23dff..1171def 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2089,6 +2089,9 @@ int kvm_dev_ioctl_check_extension(long ext)
 	case KVM_CAP_TSC_CONTROL:
 		r = kvm_has_tsc_control;
 		break;
+	case KVM_CAP_TSC_DEADLINE_TIMER:
+		r = boot_cpu_has(X86_FEATURE_TSC_DEADLINE_TIMER);
+		break;
 	default:
 		r = 0;
 		break;
diff --git a/include/linux/kvm.h b/include/linux/kvm.h
index c3892fc..68e67e5 100644
--- a/include/linux/kvm.h
+++ b/include/linux/kvm.h
@@ -557,6 +557,7 @@ struct kvm_ppc_pvinfo {
 #define KVM_CAP_MAX_VCPUS 66       /* returns max vcpus per vm */
 #define KVM_CAP_PPC_PAPR 68
 #define KVM_CAP_S390_GMAP 71
+#define KVM_CAP_TSC_DEADLINE_TIMER 72
 
 #ifdef KVM_CAP_IRQ_ROUTING
 
-- 
1.7.7.1