[PATCH v2 48/66] KVM: x86: Remove stateful CPUID handling

Sean Christopherson
2020-03-03T00:01:57Z

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>
---
Documentation/virt/kvm/api.rst | 22 ++--------
arch/x86/kvm/cpuid.c | 73 ++++++----------------------------
2 files changed, 17 insertions(+), 78 deletions(-)

diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
index ebd383fba939..c38cd9f88237 100644
--- a/Documentation/virt/kvm/api.rst
+++ b/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:

diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index b5dce17c070f..49527dbcc90c 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -495,25 +495,16 @@ 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:
@@ -894,58 +885,20 @@ 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);

--
2.24.1