KVM: arm64: Trim guest debug exception handling

The switch-case for handling guest debug exception covers
all the debug exception classes, but functionally, doesn't
do anything with them other than ESR_ELx_EC_WATCHPT_LOW.
Moreover, even though handled well, the 'default' case
could be confusing from a security point of view, stating
that the guests' actions can potentially flood the syslog.
But in reality, the code is unreachable.

Hence, trim down the function to only handle the case with
ESR_ELx_EC_WATCHPT_LOW with a simple 'if' check.
Suggested-by: Marc Zyngier <maz@kernel.org>
Signed-off-by: Raghavendra Rao Ananta <rananta@google.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20210823223940.1878930-1-rananta@google.com

arch/arm64/kvm/handle_exit.c

@@ -113,34 +113,20 @@ static int kvm_handle_wfx(struct kvm_vcpu *vcpu)
  * guest and host are using the same debug facilities it will be up to
  * userspace to re-inject the correct exception for guest delivery.
  *
- * @return: 0 (while setting vcpu->run->exit_reason), -1 for error
+ * @return: 0 (while setting vcpu->run->exit_reason)
  */
 static int kvm_handle_guest_debug(struct kvm_vcpu *vcpu)
 {
 	struct kvm_run *run = vcpu->run;
 	u32 esr = kvm_vcpu_get_esr(vcpu);
-	int ret = 0;
 
 	run->exit_reason = KVM_EXIT_DEBUG;
 	run->debug.arch.hsr = esr;
 
-	switch (ESR_ELx_EC(esr)) {
-	case ESR_ELx_EC_WATCHPT_LOW:
+	if (ESR_ELx_EC(esr) == ESR_ELx_EC_WATCHPT_LOW)
 		run->debug.arch.far = vcpu->arch.fault.far_el2;
-		fallthrough;
-	case ESR_ELx_EC_SOFTSTP_LOW:
-	case ESR_ELx_EC_BREAKPT_LOW:
-	case ESR_ELx_EC_BKPT32:
-	case ESR_ELx_EC_BRK64:
-		break;
-	default:
-		kvm_err("%s: un-handled case esr: %#08x\n",
-			__func__, (unsigned int) esr);
-		ret = -1;
-		break;
-	}
 
-	return ret;
+	return 0;
 }
 
 static int kvm_handle_unknown_ec(struct kvm_vcpu *vcpu)