KVM: Clarify KVM_{SET,GET}_ONE_REG error code documentation

The current error code documentation for KVM_GET_ONE_REG and
KVM_SET_ONE_REG could be read as implying that all architectures
implement these error codes, or that KVM guarantees which error
code is returned in a particular situation.

Because this is not really the case, this patch waters down the
documentation explicitly to remove such guarantees.

EPERM is marked as arm64-specific, since for now arm64 really is
the only architecture that yields this error code for the
finalization-required case.  Keeping this as a distinct error code
is useful however for debugging due to the statefulness of the API
in this instance.

No functional change.

Suggested-by: Andrew Jones <drjones@redhat.com>
Fixes: 395f562 ("KVM: Document errors for KVM_GET_ONE_REG and KVM_SET_ONE_REG")
Fixes: 50036ad ("KVM: arm64/sve: Document KVM API extensions for SVE")
Signed-off-by: Dave Martin <Dave.Martin@arm.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

Documentation/virtual/kvm/api.txt

@@ -1873,8 +1873,10 @@ Parameters: struct kvm_one_reg (in)
 Returns: 0 on success, negative value on failure
 Errors:
   ENOENT:   no such register
-  EPERM:    register access forbidden for architecture-dependent reasons
-  EINVAL:   other errors, such as bad size encoding for a known register
+  EINVAL:   invalid register ID, or no such register
+  EPERM:    (arm64) register access not allowed before vcpu finalization
+(These error codes are indicative only: do not rely on a specific error
+code being returned in a specific situation.)
 
 struct kvm_one_reg {
        __u64 id;
@@ -2260,10 +2262,12 @@ Architectures: all
 Type: vcpu ioctl
 Parameters: struct kvm_one_reg (in and out)
 Returns: 0 on success, negative value on failure
-Errors:
+Errors include:
   ENOENT:   no such register
-  EPERM:    register access forbidden for architecture-dependent reasons
-  EINVAL:   other errors, such as bad size encoding for a known register
+  EINVAL:   invalid register ID, or no such register
+  EPERM:    (arm64) register access not allowed before vcpu finalization
+(These error codes are indicative only: do not rely on a specific error
+code being returned in a specific situation.)
 
 This ioctl allows to receive the value of a single register implemented
 in a vcpu. The register to read is indicated by the "id" field of the