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

[Rik van Riel <riel@xxxxxxxxxx>]

On x86-64 machine_to_phys looks like it should never succeed, yet I'm
guessing it must somehow be lucky...

The problem would be that the NX bit is bit 63 of the pte, meaning that
when pte_val is called we are working with a value 2^63 higher than we
should be.

#define pte_val(x)      (((x).pte & 1) ? machine_to_phys((x).pte) : \
                         (x).pte)

static inline paddr_t machine_to_phys(maddr_t machine)
{
        paddr_t phys = mfn_to_pfn(machine >> PAGE_SHIFT);
        phys = (phys << PAGE_SHIFT) | (machine & ~PAGE_MASK);
        return phys;
}

Should we mask the 'machine' variable with PHYSICAL_MASK at
some point so we cut off the NX bit and other reserved bits?

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?

--
What is important?  What you want to be true, or what is true?

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