KVM: MIPS: Add 64BIT capability

Add a new KVM_CAP_MIPS_64BIT capability to indicate that 64-bit MIPS
guests are available and supported. In this case it should still be
possible to run 32-bit guest code. If not available it won't be possible
to run 64-bit guest code and the instructions may not be available, or
the kernel may not support full context switching of 64-bit registers.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org
Cc: linux-doc@vger.kernel.org

Documentation/virtual/kvm/api.txt

@@ -4192,3 +4192,28 @@ to KVM_CREATE_VM to create a VM which utilises it.
 
 If KVM_CHECK_EXTENSION on a kvm VM handle indicates that this capability is
 available, it means that the VM is using trap & emulate.
+
+8.7 KVM_CAP_MIPS_64BIT
+
+Architectures: mips
+
+This capability indicates the supported architecture type of the guest, i.e. the
+supported register and address width.
+
+The values returned when this capability is checked by KVM_CHECK_EXTENSION on a
+kvm VM handle correspond roughly to the CP0_Config.AT register field, and should
+be checked specifically against known values (see below). All other values are
+reserved.
+
+ 0: MIPS32 or microMIPS32.
+    Both registers and addresses are 32-bits wide.
+    It will only be possible to run 32-bit guest code.
+
+ 1: MIPS64 or microMIPS64 with access only to 32-bit compatibility segments.
+    Registers are 64-bits wide, but addresses are 32-bits wide.
+    64-bit guest code may run but cannot access MIPS64 memory segments.
+    It will also be possible to run 32-bit guest code.
+
+ 2: MIPS64 or microMIPS64 with access to all address segments.
+    Both registers and addresses are 64-bits wide.
+    It will be possible to run 64-bit or 32-bit guest code.

include/uapi/linux/kvm.h

@@ -889,6 +889,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_IMMEDIATE_EXIT 136
 #define KVM_CAP_MIPS_VZ 137
 #define KVM_CAP_MIPS_TE 138
+#define KVM_CAP_MIPS_64BIT 139
 
 #ifdef KVM_CAP_IRQ_ROUTING