Re: [PATCH] supporting non-NX/XD capable processors on x86_64 xenlinux

[Jerone Young <jyoung5@us.ibm.com>]

On Thu, 2005-08-18 at 16:09 +0100, Keir Fraser wrote:
> On 18 Aug 2005, at 15:15, Nakajima, Jun wrote:
> 
> >
> > One obvious reason is: xen-specific drivers are using __PAGE_KERNEL 
> > (and
> > they don't exist in the native Linux). If xen_contig_memory, for
> > example, does not work, it won't boot... Fixing xen_contig_memory did
> > not fix the booting problem as long as I remember?
> 
> Yeah, I see what you mean. I've just applied a patch that cleans up the 
> few remaining bad spots where we weren't using pfn_pte() or 
> pfn_pte_ma() constructors. I can now boot okay even with NX/XD disabled 
> in Xen.
> 
> By the way -- from what I've seen of this NX/XD discussion so far, it 
> sounds like even if CPUID says that NX/XD is supported, and you set the 
> EFER.NXE bit, the BIOS can still cause this to be ignored!

If I remember correctly (will create a patch shortly to make 100% sure).
If the option is off in the BIOS than NX bit will not show up as a CPU
feature flag. And of course once it's on in the bios then it does.

>  Is this true 
> and, if so, what will happen if you try to use NX/XD anyway? i.e., will 
> the bit be ignored, or will its use cause a page fault?
> 
>   -- Keir
> 
> 
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-- 
Jerone Young
IBM Linux Technology Center
jyoung5@us.ibm.com
512-838-1157 (T/L: 678-1157)