Re: [PATCH 09/31] x86/entry/32: Leave the kernel via trampoline stack

[Linus Torvalds]

On Fri, Feb 9, 2018 at 11:02 AM, Joerg Roedel <jroedel@xxxxxxx> wrote:
>
> Okay, I used movsb because I remembered that being the recommendation
> for the most efficient memcpy, and it safes me an instruction. But that
> is probably only true on modern CPUs.

Yeah, it's only true on the very latest uarchs, and even there it's
not perfect for small copies.

On the older machines that are relevant for 32-bit code, it's often
tens of cycles just for the ucode overhead, I think, and "rep movsb"
actually does things literally a byte at a time.

              Linus