Messages in this thread | | | Date | Fri, 12 Jan 2018 15:43:32 +0100 | From | Pavel Machek <> | Subject | Re: Linux 4.15-rc7 |
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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 [unhandled content-type:application/pgp-signature] | |