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Re: [Xen-devel] [PATCH] supporting non-NX/XD capable processors on x86_6

To: Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH] supporting non-NX/XD capable processors on x86_64 xenlinux
From: Jerone Young <jyoung5@xxxxxxxxxx>
Date: Thu, 18 Aug 2005 10:39:09 -0500
Cc: Jerone Young <jerone@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx, "Nakajima, Jun" <jun.nakajima@xxxxxxxxx>
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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
512-838-1157 (T/L: 678-1157)

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