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Re: kernel-PAE and NX (No eXecute)



On Wed, Nov 14, 2007 at 10:00:09AM +0100, Till Maas wrote:
 > On Mi November 14 2007, Dave Jones wrote:
 > > On Mon, Nov 12, 2007 at 08:08:08PM +0100, Till Maas wrote:
 > 
 > >  > I read
 > >  > that the 2.6.23 kernel supports NX without the need to activate 64G
 > >  > memory support?
 > >
 > > not true.
 > 
 > Here is a kernel git changelog entry that says it is true:
 > 
 > | i386: divorce CONFIG_X86_PAE from CONFIG_HIGHMEM64G
 > | 
 > | PAE is useful for more than supporting more than 4GB RAM.  It supports
 > | expanded swapspace and NX executable protections.  Some users may want NX
 > | or expanded swapspace support without the overhead or instability of
 > | highmem.  For these reasons, the following patch divorces CONFIG_X86_PAE
 > | from CONFIG_HIGHMEM64G.
 > 
 > http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=c673f1a9d994de501b674b2bb6a48bd5e912afe0

That's a really terrible exaggerated changelog entry.
The 'overhead' is still there because you need to use 3-level 64bit pagetables.
As for 'instability', well if there are bugs, how about we fix them?
(crazy idea I know, it'll never catch on).

So all this really buys you is the inability to use memory past 4G
if present.  For people with <4G, it's no different. They still
need PAE enabled to use NX.

	Dave

-- 
http://www.codemonkey.org.uk


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