linux-kernel - Re: [PATCH 00/31 v2] PTI support for x86

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Date:   Sat, 10 Feb 2018 10:15:44 +0100
From:   Adam Borowski <kilobyte@...band.pl>
To:     Joerg Roedel <jroedel@...e.de>
Cc:     Andy Lutomirski <luto@...nel.org>, Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>, Jiri Kosina <jkosina@...e.cz>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Brian Gerst <brgerst@...il.com>,
        David Laight <David.Laight@...lab.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Eduardo Valentin <eduval@...zon.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Will Deacon <will.deacon@....com>,
        "Liguori, Anthony" <aliguori@...zon.com>,
        Daniel Gruss <daniel.gruss@...k.tugraz.at>,
        Hugh Dickins <hughd@...gle.com>,
        Kees Cook <keescook@...gle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Waiman Long <llong@...hat.com>, Pavel Machek <pavel@....cz>
Subject: Re: [PATCH 00/31 v2] PTI support for x86_32

On Fri, Feb 09, 2018 at 08:11:12PM +0100, Joerg Roedel wrote:
> On Fri, Feb 09, 2018 at 05:47:43PM +0000, Andy Lutomirski wrote:
> > One thing worth noting is that performance of this whole series is
> > going to be abysmal due to the complete lack of 32-bit PCID.  Maybe
> > any kernel built with this option set that runs on a CPU that has the
> > PCID bit set in CPUID should print a big fat warning like "WARNING:
> > you are using 32-bit PTI on a 64-bit PCID-capable CPU.  Your
> > performance will increase dramatically if you switch to a 64-bit
> > kernel."
> 
> Thanks for your review. I can add this warning, but I just hope that not
> a lot of people will actually see it :)

Alas, we got some data:
https://popcon.debian.org/ says 20% of x86 users have i386 as their main ABI
(current; people with popcon installed).

Of those, 80% use 32-bit kernels: i686 881, x86_64 229, i586 14
(uname -m included in bug reports; data for 2016) -- and bug reporters tend
to have more clue than the average user.  There's no way so many folks still
use pre-2004 computers.

Thus, if you could include that big fat warning, distro developers would be
thankful.  Make it show fiery letters if you can.  Preferably, we'd want a
huge mallet reach out of the screen and bonk the user on the head, but with
that impossible, a scary message would help.

Let them use 32-bit userland, but if someone runs such a kernel on a modern
machine, some kind of verbal abuse is warranted.


Meow!
-- 
⢀⣴⠾⠻⢶⣦⠀ 
⣾⠁⢰⠒⠀⣿⡁ Vat kind uf sufficiently advanced technology iz dis!?
⢿⡄⠘⠷⠚⠋⠀                                 -- Genghis Ht'rok'din
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