KVM: x86: Do not return host topology information from KVM_GET_SUPPORTED_CPUID

Passing the host topology to the guest is almost certainly wrong
and will confuse the scheduler.  In addition, several fields of
these CPUID leaves vary on each processor; it is simply impossible to
return the right values from KVM_GET_SUPPORTED_CPUID in such a way that
they can be passed to KVM_SET_CPUID2.

The values that will most likely prevent confusion are all zeroes.
Userspace will have to override it anyway if it wishes to present a
specific topology to the guest.

Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Documentation/virt/kvm/api.rst

@@ -8310,6 +8310,20 @@ CPU[EAX=1]:ECX[24] (TSC_DEADLINE) is not reported by ``KVM_GET_SUPPORTED_CPUID``
 It can be enabled if ``KVM_CAP_TSC_DEADLINE_TIMER`` is present and the kernel
 has enabled in-kernel emulation of the local APIC.
 
+CPU topology
+~~~~~~~~~~~~
+
+Several CPUID values include topology information for the host CPU:
+0x0b and 0x1f for Intel systems, 0x8000001e for AMD systems.  Different
+versions of KVM return different values for this information and userspace
+should not rely on it.  Currently they return all zeroes.
+
+If userspace wishes to set up a guest topology, it should be careful that
+the values of these three leaves differ for each CPU.  In particular,
+the APIC ID is found in EDX for all subleaves of 0x0b and 0x1f, and in EAX
+for 0x8000001e; the latter also encodes the core id and node id in bits
+7:0 of EBX and ECX respectively.
+
 Obsolete ioctls and capabilities
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

arch/x86/kvm/cpuid.c

@@ -770,16 +770,22 @@ struct kvm_cpuid_array {
 	int nent;
 };
 
+static struct kvm_cpuid_entry2 *get_next_cpuid(struct kvm_cpuid_array *array)
+{
+	if (array->nent >= array->maxnent)
+		return NULL;
+
+	return &array->entries[array->nent++];
+}
+
 static struct kvm_cpuid_entry2 *do_host_cpuid(struct kvm_cpuid_array *array,
 					      u32 function, u32 index)
 {
-	struct kvm_cpuid_entry2 *entry;
+	struct kvm_cpuid_entry2 *entry = get_next_cpuid(array);
 
-	if (array->nent >= array->maxnent)
+	if (!entry)
 		return NULL;
 
-	entry = &array->entries[array->nent++];
-
 	memset(entry, 0, sizeof(*entry));
 	entry->function = function;
 	entry->index = index;
@@ -956,22 +962,13 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 		entry->edx = edx.full;
 		break;
 	}
-	/*
-	 * Per Intel's SDM, the 0x1f is a superset of 0xb,
-	 * thus they can be handled by common code.
-	 */
 	case 0x1f:
 	case 0xb:
 		/*
-		 * Populate entries until the level type (ECX[15:8]) of the
-		 * previous entry is zero.  Note, CPUID EAX.{0x1f,0xb}.0 is
-		 * the starting entry, filled by the primary do_host_cpuid().
+		 * No topology; a valid topology is indicated by the presence
+		 * of subleaf 1.
 		 */
-		for (i = 1; entry->ecx & 0xff00; ++i) {
-			entry = do_host_cpuid(array, function, i);
-			if (!entry)
-				goto out;
-		}
+		entry->eax = entry->ebx = entry->ecx = 0;
 		break;
 	case 0xd: {
 		u64 permitted_xcr0 = kvm_caps.supported_xcr0 & xstate_get_guest_group_perm();
@@ -1202,6 +1199,9 @@ static inline int __do_cpuid_func(struct kvm_cpuid_array *array, u32 function)
 		entry->ebx = entry->ecx = entry->edx = 0;
 		break;
 	case 0x8000001e:
+		/* Do not return host topology information.  */
+		entry->eax = entry->ebx = entry->ecx = 0;
+		entry->edx = 0; /* reserved */
 		break;
 	case 0x8000001F:
 		if (!kvm_cpu_cap_has(X86_FEATURE_SEV)) {