linux-kernel - Re: [PATCH v3 2/6] x86/uaccess: Avoid barrier_nospec() in 64-bit __get

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Message-ID: <CAHk-=wgLCzEwa=S4hZFGeOPjix-1_fDrsqR-QLaBcDM-fgkvhw@mail.gmail.com>
Date: Fri, 15 Nov 2024 17:27:06 -0800
From: Linus Torvalds <torvalds@...ux-foundation.org>
To: Josh Poimboeuf <jpoimboe@...nel.org>
Cc: David Laight <David.Laight@...lab.com>, "x86@...nel.org" <x86@...nel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Thomas Gleixner <tglx@...utronix.de>, 
	Borislav Petkov <bp@...en8.de>, Peter Zijlstra <peterz@...radead.org>, 
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Waiman Long <longman@...hat.com>, 
	Dave Hansen <dave.hansen@...ux.intel.com>, Ingo Molnar <mingo@...hat.com>, 
	Michael Ellerman <mpe@...erman.id.au>, 
	"linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>, Andrew Cooper <andrew.cooper3@...rix.com>, 
	Mark Rutland <mark.rutland@....com>, "Kirill A . Shutemov" <kirill@...temov.name>
Subject: Re: [PATCH v3 2/6] x86/uaccess: Avoid barrier_nospec() in 64-bit __get_user()

On Fri, 15 Nov 2024 at 15:06, Josh Poimboeuf <jpoimboe@...nel.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