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Message-ID: <1365555234.25498.91.camel@gandalf.local.home>
Date: Tue, 09 Apr 2013 20:53:54 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: "H. Peter Anvin" <hpa@...or.com>
Cc: Eric Northup <digitaleric@...gle.com>, Kees Cook
<keescook@...omium.org>, "kernel-hardening@...ts.openwall.com"
<kernel-hardening@...ts.openwall.com>, Ingo Molnar <mingo@...hat.com>, LKML
<linux-kernel@...r.kernel.org>, "x86@...nel.org" <x86@...nel.org>, Konrad
Rzeszutek Wilk <konrad.wilk@...cle.com>, Jeremy Fitzhardinge
<jeremy@...p.org>, Marcelo Tosatti <mtosatti@...hat.com>, Alex Shi
<alex.shi@...el.com>, Borislav Petkov <borislav.petkov@....com>, Alexander
Duyck <alexander.h.duyck@...el.com>, Frederic Weisbecker
<fweisbec@...il.com>, "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
"virtualization@...ts.linux-foundation.org"
<virtualization@...ts.linux-foundation.org>, Dan Rosenberg
<drosenberg@...curity.com>, Julien Tinnes <jln@...gle.com>, Will Drewry
<wad@...omium.org>
Subject: Re: Readonly GDT
On Tue, 2013-04-09 at 17:43 -0700, H. Peter Anvin wrote:
> OK, thinking about the GDT here.
>
> The GDT is quite small -- 256 bytes on i386, 128 bytes on x86-64. As
> such, we probably don't want to allocate a full page to it for only
> that. This means that in order to create a readonly mapping we have to
> pack GDTs from different CPUs together in the same pages, *or* we
> tolerate that other things on the same page gets reflected in the same
> mapping.
What about grouping via nodes?
>
> However, the packing solution has the advantage of reducing address
> space consumption which matters on 32 bits: even on i386 we can easily
> burn a megabyte of address space for 4096 processors, but burning 16
> megabytes starts to hurt.
Having 4096 32 bit processors, you deserve what you get. ;-)
-- Steve
>
> It would be important to measure the performance impact on task switch,
> though.
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