KVM: x86: Remove stateful CPUID handling

Remove the code for handling stateful CPUID 0x2 and mark the associated
flags as deprecated.  WARN if host CPUID 0x2.0.AL > 1, i.e. if by some
miracle a host with stateful CPUID 0x2 is encountered.

No known CPU exists that supports hardware accelerated virtualization
_and_ a stateful CPUID 0x2.  Barring an extremely contrived nested
virtualization scenario, stateful CPUID support is dead code.
Suggested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Suggested-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/api.rst

@@ -1574,8 +1574,8 @@ This ioctl would set vcpu's xcr to the value userspace specified.
   };
 
   #define KVM_CPUID_FLAG_SIGNIFCANT_INDEX		BIT(0)
-  #define KVM_CPUID_FLAG_STATEFUL_FUNC		BIT(1)
-  #define KVM_CPUID_FLAG_STATE_READ_NEXT		BIT(2)
+  #define KVM_CPUID_FLAG_STATEFUL_FUNC		BIT(1) /* deprecated */
+  #define KVM_CPUID_FLAG_STATE_READ_NEXT		BIT(2) /* deprecated */
 
   struct kvm_cpuid_entry2 {
 	__u32 function;
@@ -1626,13 +1626,6 @@ emulate them efficiently. The fields in each entry are defined as follows:
 
         KVM_CPUID_FLAG_SIGNIFCANT_INDEX:
            if the index field is valid
-        KVM_CPUID_FLAG_STATEFUL_FUNC:
-           if cpuid for this function returns different values for successive
-           invocations; there will be several entries with the same function,
-           all with this flag set
-        KVM_CPUID_FLAG_STATE_READ_NEXT:
-           for KVM_CPUID_FLAG_STATEFUL_FUNC entries, set if this entry is
-           the first entry to be read by a cpu
 
    eax, ebx, ecx, edx:
          the values returned by the cpuid instruction for
@@ -3347,8 +3340,8 @@ The member 'flags' is used for passing flags from userspace.
 ::
 
   #define KVM_CPUID_FLAG_SIGNIFCANT_INDEX		BIT(0)
-  #define KVM_CPUID_FLAG_STATEFUL_FUNC		BIT(1)
-  #define KVM_CPUID_FLAG_STATE_READ_NEXT		BIT(2)
+  #define KVM_CPUID_FLAG_STATEFUL_FUNC		BIT(1) /* deprecated */
+  #define KVM_CPUID_FLAG_STATE_READ_NEXT		BIT(2) /* deprecated */
 
   struct kvm_cpuid_entry2 {
 	__u32 function;
@@ -3394,13 +3387,6 @@ The fields in each entry are defined as follows:
 
         KVM_CPUID_FLAG_SIGNIFCANT_INDEX:
            if the index field is valid
-        KVM_CPUID_FLAG_STATEFUL_FUNC:
-           if cpuid for this function returns different values for successive
-           invocations; there will be several entries with the same function,
-           all with this flag set
-        KVM_CPUID_FLAG_STATE_READ_NEXT:
-           for KVM_CPUID_FLAG_STATEFUL_FUNC entries, set if this entry is
-           the first entry to be read by a cpu
 
    eax, ebx, ecx, edx:
 

arch/x86/kvm/cpuid.c

@@ -490,25 +490,16 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 		 * time, with the least-significant byte in EAX enumerating the
 		 * number of times software should do CPUID(2, 0).
 		 *
-		 * Modern CPUs (quite likely every CPU KVM has *ever* run on)
-		 * are less idiotic.  Intel's SDM states that EAX & 0xff "will
-		 * always return 01H. Software should ignore this value and not
+		 * Modern CPUs, i.e. every CPU KVM has *ever* run on are less
+		 * idiotic.  Intel's SDM states that EAX & 0xff "will always
+		 * return 01H. Software should ignore this value and not
 		 * interpret it as an informational descriptor", while AMD's
 		 * APM states that CPUID(2) is reserved.
+		 *
+		 * WARN if a frankenstein CPU that supports virtualization and
+		 * a stateful CPUID.0x2 is encountered.
 		 */
-		max_idx = entry->eax & 0xff;
-		if (likely(max_idx <= 1))
-			break;
-
-		entry->flags |= KVM_CPUID_FLAG_STATEFUL_FUNC;
-		entry->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
-
-		for (i = 1; i < max_idx; ++i) {
-			entry = do_host_cpuid(array, function, 0);
-			if (!entry)
-				goto out;
-			entry->flags |= KVM_CPUID_FLAG_STATEFUL_FUNC;
-		}
+		WARN_ON_ONCE((entry->eax & 0xff) > 1);
 		break;
 	/* functions 4 and 0x8000001d have additional index. */
 	case 4:
@@ -892,58 +883,20 @@ int kvm_dev_ioctl_get_cpuid(struct kvm_cpuid2 *cpuid,
 	return r;
 }
 
-static int move_to_next_stateful_cpuid_entry(struct kvm_vcpu *vcpu, int i)
-{
-	struct kvm_cpuid_entry2 *e = &vcpu->arch.cpuid_entries[i];
-	struct kvm_cpuid_entry2 *ej;
-	int j = i;
-	int nent = vcpu->arch.cpuid_nent;
-
-	e->flags &= ~KVM_CPUID_FLAG_STATE_READ_NEXT;
-	/* when no next entry is found, the current entry[i] is reselected */
-	do {
-		j = (j + 1) % nent;
-		ej = &vcpu->arch.cpuid_entries[j];
-	} while (ej->function != e->function);
-
-	ej->flags |= KVM_CPUID_FLAG_STATE_READ_NEXT;
-
-	return j;
-}
-
-/* find an entry with matching function, matching index (if needed), and that
- * should be read next (if it's stateful) */
-static int is_matching_cpuid_entry(struct kvm_cpuid_entry2 *e,
-	u32 function, u32 index)
-{
-	if (e->function != function)
-		return 0;
-	if ((e->flags & KVM_CPUID_FLAG_SIGNIFCANT_INDEX) && e->index != index)
-		return 0;
-	if (unlikely(e->flags & KVM_CPUID_FLAG_STATEFUL_FUNC) &&
-	    !(e->flags & KVM_CPUID_FLAG_STATE_READ_NEXT))
-		return 0;
-	return 1;
-}
-
 struct kvm_cpuid_entry2 *kvm_find_cpuid_entry(struct kvm_vcpu *vcpu,
 					      u32 function, u32 index)
 {
+	struct kvm_cpuid_entry2 *e;
 	int i;
-	struct kvm_cpuid_entry2 *best = NULL;
 
 	for (i = 0; i < vcpu->arch.cpuid_nent; ++i) {
-		struct kvm_cpuid_entry2 *e;
-
 		e = &vcpu->arch.cpuid_entries[i];
-		if (is_matching_cpuid_entry(e, function, index)) {
-			if (unlikely(e->flags & KVM_CPUID_FLAG_STATEFUL_FUNC))
-				move_to_next_stateful_cpuid_entry(vcpu, i);
-			best = e;
-			break;
-		}
+
+		if (e->function == function && (e->index == index ||
+		    !(e->flags & KVM_CPUID_FLAG_SIGNIFCANT_INDEX)))
+			return e;
 	}
-	return best;
+	return NULL;
 }
 EXPORT_SYMBOL_GPL(kvm_find_cpuid_entry);