Date: Sat, 7 Jan 2017 08:45:37 +0100 From: Ingo Molnar <mingo@...nel.org> To: Andy Lutomirski <luto@...nel.org> Cc: Thomas Garnier <thgarnie@...gle.com>, Arjan van de Ven <arjan@...ux.intel.com>, Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, "H . Peter Anvin" <hpa@...or.com>, Kees Cook <keescook@...omium.org>, Borislav Petkov <bp@...en8.de>, Dave Hansen <dave@...1.net>, Chen Yucong <slaoub@...il.com>, Paul Gortmaker <paul.gortmaker@...driver.com>, Andrew Morton <akpm@...ux-foundation.org>, Masahiro Yamada <yamada.masahiro@...ionext.com>, Sebastian Andrzej Siewior <bigeasy@...utronix.de>, Anna-Maria Gleixner <anna-maria@...utronix.de>, Boris Ostrovsky <boris.ostrovsky@...cle.com>, Rasmus Villemoes <linux@...musvillemoes.dk>, Michael Ellerman <mpe@...erman.id.au>, Juergen Gross <jgross@...e.com>, Richard Weinberger <richard@....at>, X86 ML <x86@...nel.org>, "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com> Subject: Re: [RFC] x86/mm/KASLR: Remap GDTs at fixed location * Andy Lutomirski <luto@...nel.org> wrote: > > I looked back at the fixmap, and I can see a way it could be done (using > > NR_CPUS) like the other fixmap ranges. It would limit the number of cpus to > > 512 (there is 2M memory left on fixmap on the default configuration). That's > > if we never add any other fixmap on x64. I don't know if it is an acceptable > > number and if the fixmap region could be increased. (128 if we do your kvm > > trick, of course). > > IIRC we need 4096 CPUs. On 64-bit the limit is 8192 CPUs, and the SGI guys are relying on that up to the tune of 6144 cores already, and I'd say the 64-bit CPU count is likely to go up further with 5-level paging. On 32-bit the reasonable CPU limit is the number that the Intel 32-bit cluster computing nodes use. The latest public numbers are I think 36 'tiles' with each tile being a 2-CPU SMT core - i.e. a limit of 72 CPUs has to be maintained. (They'll obviously go to 64-bit as well so this problem will go away in a hardware generation or two.) So I'd say 128 CPUs on 32-bit should be a reasonable practical limit going forward. Right now our 32-bit limit is 512 CPUs IIRC, but I don't think any real hardware in production is reaching that. > P.S. Let's do the move to the fixmap, read/write as a separate patch. That will > make bisecting much easier. Absolutely, but this has to be within the same series, as the interim fixmap-only step is less secure in some circumstances: we are moving the writable GDT from a previously randomized location to a fixed location. Thanks, Ingo
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