linux-kernel - Re: Readonly GDT

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Date:	Tue, 09 Apr 2013 17:58:34 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Steven Rostedt <rostedt@...dmis.org>
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 04/09/2013 05:53 PM, Steven Rostedt wrote:
> 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?
> 

Would be nicer for locality, although probably adds [even] more complexity.

We don't really care about 32-bit NUMA anymore -- it keeps getting
suggested for deletion, even.  For 64-bit it might make sense to just
reflect out of the percpu area even though it munches address space.

>>
>> 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. ;-)
> 

Well, the main problem is that it might get difficult to make this a
runtime thing; it more likely ends up being a compile-time bit.

	-hpa


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