Re: [Xen-devel] x86-64 machine_to_phys vs NX bit

["Jan Beulich" <jbeulich@xxxxxxxxxx>]

>>> Keir Fraser <Keir.Fraser@xxxxxxxxxxxx> 25.08.06 09:32 >>>
>On 24/8/06 8:25 pm, "Rik van Riel" <riel@xxxxxxxxxx> wrote:
>
>> Say, something like the following?
>> 
>> -    paddr_t phys = mfn_to_pfn(machine >> PAGE_SHIFT);
>> +    paddr_t phys = mfn_to_pfn((machine >> PAGE_SHIFT) & PHYSICAL_MASK);
>> 
>> I'm still thinking I may have missed something in the code
>> somewhere, but I've been looking at this for over an hour now
>> and can't seem to find it...
>> 
>> Any ideas?
>
>Your suggested patch looks reasonable but it'd be good to find out why this

I'd suggest not changing machine_to_phys(), but its callers (where needed), in
order to prevent hiding errors.

>hasn't caused us problems. For example, perhaps supported_pte_mask doesn't
>include PAGE_NX, so we're never setting the NX bit on 64-bit PTEs? That must

__supported_pte_mask is all ones on a system I just checked this on; pte_val()
for a page with _PAGE_NX set indeed returns (end_pfn << PAGE_SHIFT) | 
prot_flags.
With that I guess there are just too few pages making use of _PAGE_NX yet,
so the problem went unnoticed so far.

>be worth checking out, possibly also tracing machine_to_phys to find out
>where that bit 63 goes -- I agree that it looks like mfn_to_pfn() shouldn#t
>work if bit63 is set in the 'maddr' argument.

Jan

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