linux-kernel - Re: objtool clac/stac handling change..

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Message-ID: <20200703221219.GV2786714@ZenIV.linux.org.uk>
Date:   Fri, 3 Jul 2020 23:12:19 +0100
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Michael Ellerman <mpe@...erman.id.au>,
        Christophe Leroy <christophe.leroy@....fr>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: objtool clac/stac handling change..

On Fri, Jul 03, 2020 at 02:10:08PM -0700, Linus Torvalds wrote:

> Yeah, the "stac" instruction isn't hugely fast, and serializes the
> pipeline, so it's a nasty 20 cycles or something.
> 
> But for chissake, this
>  (a) happens approximately never
>  (b) is after a fault that took a thousand cycles
> 
> so the trivial thing to do is to just say "yeah, you need to add the
> STAC when your optimistic thing failed and you have to fall back to
> the byte-at-a-time tail case".

Not the problem I'm concerned about, really.  However, I would really
like to lift stac/clac into the *callers* of raw_copy_from_user()
et.al. and fold them into user_access_begin/user_access_end there.

And that's where the rules become very interesting - raw_copy_from_user()
is not "succeed or fail" thing, it's "tell me how much has been left
to copy" one.  Put it that way - here we really do have outputs on
fault.

PS: I hope to kill __copy_from_user()/__copy_to_user() outside of
arch/* this cycle; not much is left by now.  So I'm not talking about
lifting stac/clac out into the wild - it will merge with access_ok
into user_access_begin/end.