MIPS: KVM: Handle TRAP exceptions from guest kernel

Trap instructions are used by Linux to implement BUG_ON(), however KVM
doesn't pass trap exceptions on to the guest if they occur in guest
kernel mode, instead triggering an internal error "Exception Code: 13,
not yet handled". The guest kernel then doesn't get a chance to print
the usual BUG message and stack trace.

Implement handling of the trap exception so that it gets passed to the
guest and the user is left with a more useful log message.

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

1 files changed, 36 insertions, 0 deletions

diff --git a/arch/mips/kvm/emulate.c b/arch/mips/kvm/emulate.c
index 838d3a6a5b7d..33e132dc7de8 100644
--- a/arch/mips/kvm/emulate.c
+++ b/arch/mips/kvm/emulate.c
@@ -1970,6 +1970,41 @@ enum emulation_result kvm_mips_emulate_bp_exc(unsigned long cause,
 	return er;
 }
 
+enum emulation_result kvm_mips_emulate_trap_exc(unsigned long cause,
+						uint32_t *opc,
+						struct kvm_run *run,
+						struct kvm_vcpu *vcpu)
+{
+	struct mips_coproc *cop0 = vcpu->arch.cop0;
+	struct kvm_vcpu_arch *arch = &vcpu->arch;
+	enum emulation_result er = EMULATE_DONE;
+
+	if ((kvm_read_c0_guest_status(cop0) & ST0_EXL) == 0) {
+		/* save old pc */
+		kvm_write_c0_guest_epc(cop0, arch->pc);
+		kvm_set_c0_guest_status(cop0, ST0_EXL);
+
+		if (cause & CAUSEF_BD)
+			kvm_set_c0_guest_cause(cop0, CAUSEF_BD);
+		else
+			kvm_clear_c0_guest_cause(cop0, CAUSEF_BD);
+
+		kvm_debug("Delivering TRAP @ pc %#lx\n", arch->pc);
+
+		kvm_change_c0_guest_cause(cop0, (0xff),
+					  (T_TRAP << CAUSEB_EXCCODE));
+
+		/* Set PC to the exception entry point */
+		arch->pc = KVM_GUEST_KSEG0 + 0x180;
+
+	} else {
+		kvm_err("Trying to deliver TRAP when EXL is already set\n");
+		er = EMULATE_FAIL;
+	}
+
+	return er;
+}
+
 /* ll/sc, rdhwr, sync emulation */
 
 #define OPCODE 0xfc000000
@@ -2176,6 +2211,7 @@ enum emulation_result kvm_mips_check_privilege(unsigned long cause,
 		case T_SYSCALL:
 		case T_BREAK:
 		case T_RES_INST:
+		case T_TRAP:
 		case T_MSADIS:
 			break;