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

[Denys Vlasenko]

On 02/09/2018 08:02 PM, Joerg Roedel wrote:
> On Fri, Feb 09, 2018 at 09:05:02AM -0800, Linus Torvalds wrote:
> > On Fri, Feb 9, 2018 at 1:25 AM, Joerg Roedel <joro@xxxxxxxxxx> wrote:
> > > +
> > > +       /* Copy over the stack-frame */
> > > +       cld
> > > +       rep movsb
> >
> > Ugh. This is going to be horrendous. Maybe not noticeable on modern
> > CPU's, but the whole 32-bit code is kind of pointless on a modern CPU.
> >
> > At least use "rep movsl". If the kernel stack isn't 4-byte aligned,
> > you have issues.
>
> 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.

It's fast (copies data with full-width loads and stores,
up to 64-byte wide on latest Intel CPUs), but this kicks in only for
largish blocks. In your case, you are copying less than 100 bytes.