Re: [PATCH] x86: only use ERMS for user copies for larger sizes

[Linus Torvalds]

On Wed, Nov 21, 2018 at 9:27 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> It would be interesting to know exactly which copy it is that matters
> so much...  *inlining* the erms case might show that nicely in
> profiles.

Side note: the fact that Jens' patch (which I don't like in that form)
allegedly shrunk the resulting kernel binary would seem to indicate
that there's a *lot* of compile-time constant-sized memcpy calls that
we are missing, and that fall back to copy_user_generic().

It might be interesting to just change raw_copy_to/from_user() to
handle a lot more cases (in particular, handle cases where 'size' is
8-byte aligned). The special cases we *do* have may not be the right
ones (the 10-byte case in particular looks odd).

For example, instead of having a "if constant size is 8 bytes, do one
get/put_user()" case, we might have a "if constant size is < 64 just
unroll it into get/put_user()" calls.

                  Linus