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: Takuya Yoshikawa <yoshikawa.takuya@oss.ntt.co.jp>
Signed-off-by: Avi Kivity <avi@redhat.com>

1 files changed, 4 insertions, 2 deletions

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c91007f81660..bd3161c6daed 100644
--- a/arch/x86/kvm/x86.c
+++ b/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);