linux-kernel - Re: [PATCH 06/16] x86/mm/ldt: Reserve high address-space range for the LDT

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Date:   Wed, 17 Jan 2018 08:59:24 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     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@...ck.org,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Juergen Gross <jgross@...e.com>,
        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>, Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 06/16] x86/mm/ldt: Reserve high address-space range for
 the LDT

On Tue, Jan 16, 2018 at 02:51:45PM -0800, Andy Lutomirski wrote:
> On Tue, Jan 16, 2018 at 8:52 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> > On Tue, Jan 16, 2018 at 05:36:49PM +0100, Joerg Roedel wrote:
> >> From: Joerg Roedel <jroedel@...e.de>
> >>
> >> Reserve 2MB/4MB of address space for mapping the LDT to
> >> user-space.
> >
> > LDT is 64k, we need 2 per CPU, and NR_CPUS <= 64 on 32bit, that gives
> > 64K*2*64=8M > 2M.
> 
> If this works like it does on 64-bit, it only needs 128k regardless of
> the number of CPUs.  The LDT mapping is specific to the mm.

Ah, then I got my LDT things confused again... which is certainly
possible, we had a few too many variants back then.