[PATCH v3 2/6] x86/uaccess: Avoid barrier_nospec() in 64-bit __get_user()

[Linus Torvalds]

On Fri, 15 Nov 2024 at 15:06, Josh Poimboeuf <jpoimboe at kernel.org> wrote:
>
> It's sad that __get_user() is now slower than get_user() on x86, it kind
> of defeats the whole point!

Well, honestly, we've been trying to get away from __get_user() and
__put_user() for a long long time.

With CLAC/STAC, it's been probably a decade or two since __get_user()
and friends were actually a worthwhile optimization, so let's just
strive to get rid of the ones that matter.

So I think the thing to do is

 (a) find out which __get_user() it is that matters so much for that load

Do you have a profile somewhere?

 (b) convert them to use "unsafe_get_user()", with that whole

                if (can_do_masked_user_access())
                        from = masked_user_access_begin(from);
                else if (!user_read_access_begin(from, sizeof(*from)))
                        return -EFAULT;

     sequence before it.

And if it's just a single __get_user() (rather than a sequence of
them), just convert it to get_user().

Hmm?

                Linus