NX bit: AMD vs Intel?

[James Wilkinson]

Jurgen Kramer wrote:
> On my Intel system (Xeon with EMT64 support) which runs the 32-bit
> version of FC5 I can see that NX support is active (from dmesg):
> 
> NX (Execute Disable) protection: active
> 
> On my other system, a AMD64 system running the 64-bit version of FC4,
> there is no message regarding NX support being enabled of disabled at
> all.
> 
> Checking init.c in the kernel sources it seems that NX support is
> depended on PAE support and NX support from the processor. As the AMD
> processor supports both I aspect to see a NX procection active message
> on my AMD system as well. Does NX support suppose to work on 64-bit
> kernels ?

[james at kendrick linux-2.6.16.1]$ grep -R "NX (Execute Disable) protection" .
./arch/i386/mm/init.c:          printk("NX (Execute Disable) protection: active\n");

So you don't see the message on x86-64 because it's only actually
present in i386 code. This might be because there's a huge amount of
i386-compatible computers that *don't* support NX. It's always been
there on AMD64 (and apparently on *nearly* all Intel EM64T processors:
http://www.theregister.co.uk/2004/07/30/intel_nocona_nx/ ).

I'm not sure how Linux handles the Noconas without XD (Intel's name for
it). But I understand NX is always on when it's there.

Hope this helps,

James.

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