Re: [PATCH v7 03/14] x86/cet/ibt: Add IBT legacy code bitmap setup function

[H.J. Lu]

On Mon, Jun 10, 2019 at 3:59 PM Dave Hansen <dave.hansen@xxxxxxxxx> wrote:
>
> > We then create PR_MARK_CODE_AS_LEGACY.  The kernel will set the bitmap, but it
> > is going to be slow.
>
> Slow compared to what?  We're effectively adding one (quick) system call
> to a path that, today, has at *least* half a dozen syscalls and probably
> a bunch of page faults.  Heck, we can probably avoid the actual page
> fault to populate the bitmap if we're careful.  That alone would put a
> syscall on equal footing with any other approach.  If the bit setting
> crossed a page boundary it would probably win.
>
> > Perhaps we still let the app fill the bitmap?
>
> I think I'd want to see some performance data on it first.

Updating legacy bitmap in user space from kernel requires

long q;

get_user(q, ...);
q |= mask;
put_user(q, ...);

instead of

*p |= mask;

get_user + put_user was quite slow when we tried before.

-- 
H.J.