linux-kernel - Re: Linux 4.15-rc7

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Message-ID: <20180112144332.GA28083@amd>
Date:   Fri, 12 Jan 2018 15:43:32 +0100
From:   Pavel Machek <pavel@....cz>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Olivier Galibert <galibert@...ox.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        jikos@...e.cz
Subject: Re: Linux 4.15-rc7

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.

Too bad.

> 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/

Well, I guess at this point I'm looking for a good place to start from...

> 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 do have Core Solo here'd like to keep working (and useful for web
browsing). Then there's Pentium M. Occasionaly I run 32-bit kernels on
modern machines for testing.

Thanks,
									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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