KVM: Remove redundant reading of rax on OUT instructions

kvm_emulate_pio() and complete_pio() both read out the
RAX register value and copy it to a place into which
the value read out from the port will be copied later.

This patch removes this redundancy.

/*** snippet from arch/x86/kvm/x86.c ***/
int complete_pio(struct kvm_vcpu *vcpu)
{
	...
	if (!io->string) {
		if (io->in) {
			val = kvm_register_read(vcpu, VCPU_REGS_RAX);
			memcpy(&val, vcpu->arch.pio_data, io->size);
			kvm_register_write(vcpu, VCPU_REGS_RAX, val);
		}
	...
Signed-off-by: NTakuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: NAvi Kivity <avi@redhat.com>

arch/x86/kvm/x86.c

@@ -3551,8 +3551,10 @@ int kvm_emulate_pio(struct kvm_vcpu *vcpu, int in, int size, unsigned port)
 	trace_kvm_pio(vcpu->run->io.direction == KVM_EXIT_IO_OUT, port,
 		      size, 1);
 
-	val = kvm_register_read(vcpu, VCPU_REGS_RAX);
-	memcpy(vcpu->arch.pio_data, &val, 4);
+	if (!vcpu->arch.pio.in) {
+		val = kvm_register_read(vcpu, VCPU_REGS_RAX);
+		memcpy(vcpu->arch.pio_data, &val, 4);
+	}
 
 	if (!kernel_pio(vcpu, vcpu->arch.pio_data)) {
 		complete_pio(vcpu);