Takuya Yoshikawa wrote:
Fix the x86 emulation of in:
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);
}
...
Self explanation: reading RAX before copying pio data is for preserving
higher bits of the register. If we are sure that pio emulation ends by
complete_pio() and complete_pio() does this higher bits preservation,
we no longer need to mind preserving the higer bits of arch.pio_data
in each device's read handler: may reduce some switches and memcpies,
if we want.
Anyway, if no device's read handler depends on the memcpied 4 bytes of
RAX, I hope so, this patch seems to work and reduces extra memcpy.
Signed-off-by: Takuya Yoshikawa <yoshikawa.takuya@xxxxxxxxxxxxx>
---
arch/x86/kvm/x86.c | 6 ++++--
1 files changed, 4 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index d47ceda..fcbe3a7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3550,8 +3550,10 @@ 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);
--
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