MIPS: KVM: Sign extend MFC0/RDHWR results

When emulating MFC0 instructions to load 32-bit values from guest COP0
registers and the RDHWR instruction to read the CC (Count) register,
sign extend the result to comply with the MIPS64 architecture. The
result must be in canonical 32-bit form or the guest may malfunction.
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: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/mips/kvm/emulate.c

@@ -1072,14 +1072,15 @@ enum emulation_result kvm_mips_emulate_CP0(union mips_instruction inst,
 #endif
 			/* Get reg */
 			if ((rd == MIPS_CP0_COUNT) && (sel == 0)) {
-				vcpu->arch.gprs[rt] = kvm_mips_read_count(vcpu);
+				vcpu->arch.gprs[rt] =
+				    (s32)kvm_mips_read_count(vcpu);
 			} else if ((rd == MIPS_CP0_ERRCTL) && (sel == 0)) {
 				vcpu->arch.gprs[rt] = 0x0;
 #ifdef CONFIG_KVM_MIPS_DYN_TRANS
 				kvm_mips_trans_mfc0(inst, opc, vcpu);
 #endif
 			} else {
-				vcpu->arch.gprs[rt] = cop0->reg[rd][sel];
+				vcpu->arch.gprs[rt] = (s32)cop0->reg[rd][sel];
 
 #ifdef CONFIG_KVM_MIPS_DYN_TRANS
 				kvm_mips_trans_mfc0(inst, opc, vcpu);
@@ -2380,7 +2381,7 @@ enum emulation_result kvm_mips_handle_ri(u32 cause, u32 *opc,
 					     current_cpu_data.icache.linesz);
 			break;
 		case MIPS_HWR_CC:		/* Read count register */
-			arch->gprs[rt] = kvm_mips_read_count(vcpu);
+			arch->gprs[rt] = (s32)kvm_mips_read_count(vcpu);
 			break;
 		case MIPS_HWR_CCRES:		/* Count register resolution */
 			switch (current_cpu_data.cputype) {