From mboxrd@z Thu Jan  1 00:00:00 1970
From: "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: Readonly GDT
Date: Tue, 09 Apr 2013 17:58:34 -0700
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To: Steven Rostedt <rostedt@goodmis.org>
Cc: Alexander Duyck <alexander.h.duyck@intel.com>, Alex Shi <alex.shi@intel.com>, Jeremy Fitzhardinge <jeremy@goop.org>, Will Drewry <wad@chromium.org>, Kees Cook <keescook@chromium.org>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, "kernel-hardening@lists.openwall.com" <kernel-hardening@lists.openwall.com>, Frederic Weisbecker <fweisbec@gmail.com>, Dan Rosenberg <drosenberg@vsecurity.com>, "x86@kernel.org" <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>, Borislav Petkov <borislav.petkov@amd.com>, Ingo Molnar <mingo@redhat.com>, Julien Tinnes <jln@google.com>, "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>, "virtualization@lists.linux-foundation.org" <virtualization@lists.linux-foundation.org>, "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>
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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