KVM: x86: Add blurb to CPUID tracepoint when using max basic leaf values

Tack on "used max basic" at the end of the CPUID tracepoint when the
output values correspond to the max basic leaf, i.e. when emulating
Intel's out-of-range CPUID behavior.  Observing "cpuid entry not found"
in the tracepoint with non-zero output values is confusing for users
that aren't familiar with the out-of-range semantics, and qualifying the
"not found" case hopefully makes it clear that "found" means "found the
exact entry".

Suggested-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/cpuid.c

@@ -990,13 +990,15 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
 {
 	u32 orig_function = *eax, function = *eax, index = *ecx;
 	struct kvm_cpuid_entry2 *entry;
-	bool exact;
+	bool exact, used_max_basic = false;
 
 	entry = kvm_find_cpuid_entry(vcpu, function, index);
 	exact = !!entry;
 
-	if (!entry && !exact_only)
+	if (!entry && !exact_only) {
 		entry = get_out_of_range_cpuid_entry(vcpu, &function, index);
+		used_max_basic = !!entry;
+	}
 
 	if (entry) {
 		*eax = entry->eax;
@@ -1026,7 +1028,8 @@ bool kvm_cpuid(struct kvm_vcpu *vcpu, u32 *eax, u32 *ebx,
 			}
 		}
 	}
-	trace_kvm_cpuid(orig_function, index, *eax, *ebx, *ecx, *edx, exact);
+	trace_kvm_cpuid(orig_function, index, *eax, *ebx, *ecx, *edx, exact,
+			used_max_basic);
 	return exact;
 }
 EXPORT_SYMBOL_GPL(kvm_cpuid);

arch/x86/kvm/trace.h

@@ -153,8 +153,8 @@ TRACE_EVENT(kvm_fast_mmio,
 TRACE_EVENT(kvm_cpuid,
 	TP_PROTO(unsigned int function, unsigned int index, unsigned long rax,
 		 unsigned long rbx, unsigned long rcx, unsigned long rdx,
-		 bool found),
-	TP_ARGS(function, index, rax, rbx, rcx, rdx, found),
+		 bool found, bool used_max_basic),
+	TP_ARGS(function, index, rax, rbx, rcx, rdx, found, used_max_basic),
 
 	TP_STRUCT__entry(
 		__field(	unsigned int,	function	)
@@ -164,6 +164,7 @@ TRACE_EVENT(kvm_cpuid,
 		__field(	unsigned long,	rcx		)
 		__field(	unsigned long,	rdx		)
 		__field(	bool,		found		)
+		__field(	bool,		used_max_basic	)
 	),
 
 	TP_fast_assign(
@@ -174,12 +175,14 @@ TRACE_EVENT(kvm_cpuid,
 		__entry->rcx		= rcx;
 		__entry->rdx		= rdx;
 		__entry->found		= found;
+		__entry->used_max_basic	= used_max_basic;
 	),
 
-	TP_printk("func %x idx %x rax %lx rbx %lx rcx %lx rdx %lx, cpuid entry %s",
+	TP_printk("func %x idx %x rax %lx rbx %lx rcx %lx rdx %lx, cpuid entry %s%s",
 		  __entry->function, __entry->index, __entry->rax,
 		  __entry->rbx, __entry->rcx, __entry->rdx,
-		  __entry->found ? "found" : "not found")
+		  __entry->found ? "found" : "not found",
+		  __entry->used_max_basic ? ", used max basic" : "")
 );
 
 #define AREG(x) { APIC_##x, "APIC_" #x }