[PATCH] x86/uaccess: Avoid barrier_nospec() in copy_from_user()

[Linus Torvalds]

On Wed, 16 Oct 2024 at 15:03, Andrew Cooper <andrew.cooper3 at citrix.com> wrote:
>
> That doesn't have the same semantics, does it?

Correct. It just basically makes all positive addresses be force-canonicalized.

> If AMD think it's appropriate, then what you probably want is the real
> branch as per before (to maintain architectural user behaviour), and
> then use a trick such as this one in place of the LFENCE for speed in
> the common case.

The problem with the branch is that it really can only branch on the
sign bit - because of LAM.

So with LAM, those bits are pretty much ignored anyway.

> > So that barrier really *is* very expensive. Surprisingly so.
>
> 7% performance is what it costs to maintain the security barrier we were
> sold originally.

Absolutely. And the masking was something that basically says "we get
it all back" (with "all" being just this part, of course - never mind
all the other workarounds).

> Forgive me if I think that we (the SW people) are getting the raw end of
> the deal here, while vendors keep selling more and more expensive chips
> that don't work safely.

I'm 100% with you.

My preference would actually be to do nothing, on the assumption that
the AMD issue is actually impossible to trigger (due to CLAC/STAC
serializing memory address checks - which the timings certainly imply
they do).

But if we have to do magic bit masking, I'd rather it be *fast* magic
bit masking.

               Linus