KVM: MIPS: Implement HYPCALL emulation

Emulate the HYPCALL instruction added in the VZ ASE and used by the MIPS
paravirtualised guest support that is already merged. The new hypcall.c
handles arguments and the return value. No actual hypercalls are yet
supported, but this still allows us to safely step over hypercalls and
set an error code in the return value for forward compatibility.

Non-zero HYPCALL codes are not handled.

We also document the hypercall ABI which asm/kvm_para.h uses.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: Andreas Herrmann <andreas.herrmann@caviumnetworks.com>
Cc: David Daney <david.daney@cavium.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org
Cc: linux-doc@vger.kernel.org

arch/mips/include/asm/kvm_host.h

@@ -229,6 +229,7 @@ enum emulation_result {
 	EMULATE_WAIT,		/* WAIT instruction */
 	EMULATE_PRIV_FAIL,
 	EMULATE_EXCEPT,		/* A guest exception has been generated */
+	EMULATE_HYPERCALL,	/* HYPCALL instruction */
 };
 
 #define mips3_paddr_to_tlbpfn(x) \
@@ -832,6 +833,12 @@ unsigned int kvm_mips_config3_wrmask(struct kvm_vcpu *vcpu);
 unsigned int kvm_mips_config4_wrmask(struct kvm_vcpu *vcpu);
 unsigned int kvm_mips_config5_wrmask(struct kvm_vcpu *vcpu);
 
+/* Hypercalls (hypcall.c) */
+
+enum emulation_result kvm_mips_emul_hypcall(struct kvm_vcpu *vcpu,
+					    union mips_instruction inst);
+int kvm_mips_handle_hypcall(struct kvm_vcpu *vcpu);
+
 /* Dynamic binary translation */
 extern int kvm_mips_trans_cache_index(union mips_instruction inst,
 				      u32 *opc, struct kvm_vcpu *vcpu);

arch/mips/include/uapi/asm/inst.h

@@ -179,7 +179,7 @@ enum cop0_coi_func {
 	tlbr_op	      = 0x01, tlbwi_op	    = 0x02,
 	tlbwr_op      = 0x06, tlbp_op	    = 0x08,
 	rfe_op	      = 0x10, eret_op	    = 0x18,
-	wait_op       = 0x20,
+	wait_op       = 0x20, hypcall_op    = 0x28
 };
 
 /*