arm/arm64: KVM: avoid unnecessary guest register mangling on MMIO read

Currently we mangle the endianness of the guest's register even on an
MMIO _read_, where it is completely useless, because we will not use
the value of that register.
Rework the io_mem_abort() function to clearly separate between reads
and writes and only do the endianness mangling on MMIO writes.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>

arch/arm/kvm/mmio.c

@@ -187,15 +187,18 @@ int io_mem_abort(struct kvm_vcpu *vcpu, struct kvm_run *run,
 	}
 
 	rt = vcpu->arch.mmio_decode.rt;
-	data = vcpu_data_guest_to_host(vcpu, *vcpu_reg(vcpu, rt), mmio.len);
 
-	trace_kvm_mmio((mmio.is_write) ? KVM_TRACE_MMIO_WRITE :
-					 KVM_TRACE_MMIO_READ_UNSATISFIED,
-			mmio.len, fault_ipa,
-			(mmio.is_write) ? data : 0);
+	if (mmio.is_write) {
+		data = vcpu_data_guest_to_host(vcpu, *vcpu_reg(vcpu, rt),
+					       mmio.len);
 
-	if (mmio.is_write)
+		trace_kvm_mmio(KVM_TRACE_MMIO_WRITE, mmio.len,
+			       fault_ipa, data);
 		mmio_write_buf(mmio.data, mmio.len, data);
+	} else {
+		trace_kvm_mmio(KVM_TRACE_MMIO_READ_UNSATISFIED, mmio.len,
+			       fault_ipa, 0);
+	}
 
 	if (vgic_handle_mmio(vcpu, run, &mmio))
 		return 1;