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KVM: x86: Remove stateful CPUID handling
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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>
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Sean Christopherson authored and Paolo Bonzini committed Mar 16, 2020
1 parent c571a14 commit 7ff6c03
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Showing 2 changed files with 17 additions and 78 deletions.
22 changes: 4 additions & 18 deletions Documentation/virt/kvm/api.rst
View file
Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand Down Expand Up @@ -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
Expand Down Expand Up @@ -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;
Expand Down Expand Up @@ -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:

Expand Down
73 changes: 13 additions & 60 deletions arch/x86/kvm/cpuid.c
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Original file line number Diff line number Diff line change
Expand Up @@ -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:
Expand Down Expand Up @@ -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);

Expand Down

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