On Mon, 2015-08-24 at 20:44 -0400, Stephen Powell wrote:
> On Mon, 24 Aug 2015 13:52:30 -0400 (EDT), Mirko Parthey wrote:
> > One such benefit is that the NX bit (non-executable memory pages)
> > is only available with 64 bit page table entries, which in turn depend on
> > PAE mode. This could be an argument for preferring a PAE kernel on
> > PAE-capable hardware.
[...]
> PAE may be a necessary condition for NX, but it is not a sufficient condition.
> I am presently using three 32-bit computers: one is a Pentium M (2G), one is
> a Pentium 4 (2G), and one is a Xeon (4G). All three are PAE-capable, and all
> three are presently running PAE Linux kernels. And all three display the
> following message during boot, according to "dmesg|less":
>
> Notice: NX (Execute Disable) protection missing in CPU!
>
> So, for my hardware, that argument doesn't seem to hold up. Am I missing
> something?
Wikipedia, that font of all wisdom, says of NX [1]
After AMD's decision to include this functionality in its AMD64
instruction set, Intel implemented the similar XD bit feature in
x86 processors beginning with the Pentium 4 processors based on
later iterations of the Prescott core. [...] It is only
available with the long mode (64-bit mode) and legacy Physical
Address Extension (PAE) page table formats, but not x86's
original 32-bit page table format because page table entries in
that format lack the 63rd bit used to disable/enable execution.
And of 64-bit X86 [2]
Intel subsequently began selling Intel 64-enabled Pentium 4s
using the E0 revision of the Prescott core, being sold on the
OEM market as the Pentium 4, model F. The E0 revision also adds
eXecute Disable (XD) (Intel's name for the NX bit)
So it appears that the XN/XD and 64-bit support coincide on a CPU.
Unless XN was retrofitted to 32-bit only CPUs produced for things like
the embedded market.
[1] https://en.wikipedia.org/wiki/NX_bit
[2] https://en.wikipedia.org/wiki/X86-64
--
Tixy
