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

[ Upstream commit 45e966fcca03ecdcccac7cb236e16eea38cc18af ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index 8826f80..b550f43 100644 (file)
--- a/Documentation/virt/kvm/api.rst
+++ b/Documentation/virt/kvm/api.rst
@@ -7300,6 +7300,20 @@ 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
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index 05b27b4..528437e 100644 (file)
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -567,16 +567,22 @@ 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++];
-
        entry->function = function;
        entry->index = index;
        entry->flags = 0;
@@ -755,22 +761,13 @@ 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:
                entry->eax &= supported_xcr0;
@@ -962,6 +959,9 @@ 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)) {