Re: KVM: avoid unnecessary memcpy in kvm_emulate_pio

Avi Kivity
2008-05-25T11:39:28Z

Marcelo Tosatti wrote:
> When emulating in instructions there's no point in copying the
> destination register contents to vcpu->arch.pio_data since it will be
> overwritten by the ioport handler.
>
> Also remove the ->decache_regs() call, since as you mentioned there is
> no register change to writeback.
>
> Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>
>
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e537005..f43f770 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2254,8 +2254,8 @@ int kvm_emulate_pio(struct kvm_vcpu *vcpu, struct kvm_run *run, int in,
> handler);
>
> kvm_x86_ops->cache_regs(vcpu);
> - memcpy(vcpu->arch.pio_data, &vcpu->arch.regs[VCPU_REGS_RAX], 4);
> - kvm_x86_ops->decache_regs(vcpu);
> + if (!in)
> + memcpy(vcpu->arch.pio_data, &vcpu->arch.regs[VCPU_REGS_RAX], 4);
>
> kvm_x86_ops->skip_emulated_instruction(vcpu);
>

The memcpy() compiles to just one mov instruction, so I didn't change
it. I dropped the ->decache_regs() call (as it can take 100 cycles on
Intel).

--
error compiling committee.c: too many arguments to function