Date: Fri, 12 Jan 2018 09:20:57 -0800
From: vcaputo@pengaru.com
To: Arnd Bergmann <arnd@arndb.de>
Cc: Pavel Machek <pavel@ucw.cz>, Olivier Galibert <galibert@pobox.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        jikos@suse.cz
Subject: Re: Linux 4.15-rc7
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On Fri, Jan 12, 2018 at 02:23:20PM +0100, Arnd Bergmann wrote:
> On Fri, Jan 12, 2018 at 12:06 PM, Pavel Machek <pavel@ucw.cz> wrote:
> > Hi!
> >
> >> Wasn't/Isn't the 4G/4G  memory layout for 32 bits essentially KPTI?
> >
> > Good point. Is that still supported? Was it ever?
> >
> > Umm. I seem to recall that 4G/4G layout was out of tree but never
> > merged.
> 
> I think that's correct: it was in RHEL3 and RHEL4 but never merged
> upstream.
> 
> However, there is an important difference between KPTI and X86_4G:
> The former unmaps the kernel pages from the user space page tables,
> but keeps both the linear mapping and the user pages visible in
> kernel mode, while the latter must have also unmapped user space
> pages from kernel mode, requiring a more expensive get_user/put_user
> implementation.
> 
> Kees mentioned an idea to also unmap user pages from kernel
> mode as an additional safeguard on top of KPTI, which would get
> it even closer to the X86_4G implementation:
> https://outflux.net/blog/archives/2018/01/04/smep-emulation-in-pti/
> 
> Could you be more specific which 32-bit x86 chips you have that are
> affected by Meltdown? Do you mean pre-2004 Pentiums or Core-Duo
> laptops? I would guess that Cyrix/Natsemi/AMD 6x86/MediaGX/Geode
> and AMD NexGen K6/K7 also affected by Spectre but probably not
> Meltdown, and most other 32-bit microarchitectures seem to be purely
> in-order.
> 

I have some Celeron D, 4GiB dedicated servers with a 32-bit stack.
They've proven to be very reliable boxes, and are the most affordable
baremetal x86 machines I've found.  I'd appreciate a PTI implementation
on them.

Thanks,
Vito Caputo