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

[Linus Torvalds]

On Thu, 21 Nov 2024 at 19:57, Linus Torvalds
<torvalds at linux-foundation.org> wrote:
>
> Anyway, that was a long way of saying: I really think we should just
> special-case the (few) important cases that get reported. Because any
> *big* improvements will come not from just inlining.

Looking around at the futex code some more, I note:

 - the cmpxchg case and futex ops use an explicit barrier too, which is bad

 - we'd actually be better off inlining not just the user access, but
the whole futex_get_value_locked(), because then the compiler will be
able to do CSE on the user address masking, and only do it once
(several places do multiple different futex_get_value_locked() calls).

iow, I think the fix for the futex case ends up being a patch
something like the attached.

[ Annoyingly, we need "can_do_masked_user_access()" even on x86,
because the 32-bit case doesn't do the address masking trick ]

I've only compiled it so far, about to actually boot into it. Pray for me,

               Linus
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