MIPS: KVM: Pass reserved instruction exceptions to guest

Previously a reserved instruction exception while in guest code would
cause a KVM internal error if kvm_mips_handle_ri() didn't recognise the
instruction (including a RDHWR from an unrecognised hardware register).

However the guest OS should really have the opportunity to catch the
exception so that it can take the appropriate actions such as sending a
SIGILL to the guest user process or emulating the instruction itself.

Therefore in these cases emulate a guest RI exception and only return
EMULATE_FAIL if that fails, being careful to revert the PC first in case
the exception occurred in a branch delay slot in which case the PC will
already point to the branch target.

Also turn the printk messages relating to these cases into kvm_debug
messages so that they aren't usually visible.

This allows crashme to run in the guest without killing the entire VM.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Gleb Natapov <gleb@kernel.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sanjay Lal <sanjayl@kymasys.com>
Cc: linux-mips@linux-mips.org
Cc: kvm@vger.kernel.org
Cc: stable@vger.kernel.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/mips/kvm/kvm_mips_emul.c

@@ -1571,17 +1571,17 @@ kvm_mips_handle_ri(unsigned long cause, uint32_t *opc,
 			arch->gprs[rt] = kvm_read_c0_guest_userlocal(cop0);
 #else
 			/* UserLocal not implemented */
-			er = kvm_mips_emulate_ri_exc(cause, opc, run, vcpu);
+			er = EMULATE_FAIL;
 #endif
 			break;
 
 		default:
-			printk("RDHWR not supported\n");
+			kvm_debug("RDHWR %#x not supported @ %p\n", rd, opc);
 			er = EMULATE_FAIL;
 			break;
 		}
 	} else {
-		printk("Emulate RI not supported @ %p: %#x\n", opc, inst);
+		kvm_debug("Emulate RI not supported @ %p: %#x\n", opc, inst);
 		er = EMULATE_FAIL;
 	}
 
@@ -1590,6 +1590,7 @@ kvm_mips_handle_ri(unsigned long cause, uint32_t *opc,
 	 */
 	if (er == EMULATE_FAIL) {
 		vcpu->arch.pc = curr_pc;
+		er = kvm_mips_emulate_ri_exc(cause, opc, run, vcpu);
 	}
 	return er;
 }