Date: Wed, 17 Jan 2018 08:59:24 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Andy Lutomirski <luto@kernel.org>
Cc: Joerg Roedel <joro@8bytes.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        "H . Peter Anvin" <hpa@zytor.com>, X86 ML <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, linux-mm@kvack.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Juergen Gross <jgross@suse.com>,
        Borislav Petkov <bp@alien8.de>, Jiri Kosina <jkosina@suse.cz>,
        Boris Ostrovsky <boris.ostrovsky@oracle.com>,
        Brian Gerst <brgerst@gmail.com>,
        David Laight <David.Laight@aculab.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        Eduardo Valentin <eduval@amazon.com>,
        Greg KH <gregkh@linuxfoundation.org>,
        Will Deacon <will.deacon@arm.com>,
        "Liguori, Anthony" <aliguori@amazon.com>,
        Daniel Gruss <daniel.gruss@iaik.tugraz.at>,
        Hugh Dickins <hughd@google.com>,
        Kees Cook <keescook@google.com>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Waiman Long <llong@redhat.com>, Joerg Roedel <jroedel@suse.de>
Subject: Re: [PATCH 06/16] x86/mm/ldt: Reserve high address-space range for
 the LDT
Message-ID: <20180117075924.GI2228@hirez.programming.kicks-ass.net>
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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@infradead.org> wrote:
> > On Tue, Jan 16, 2018 at 05:36:49PM +0100, Joerg Roedel wrote:
> >> From: Joerg Roedel <jroedel@suse.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.