Re: [Xen-devel] asm/page.h inconsistencies?

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 6 Jan 2006, at 13:29, Jan Beulich wrote:

> Could anyone explain the non-symmetric implementation of
> __pte/__pmd/__pud/__pgd compared to pte_val/pmd_val/pud_val/pgd_val on
> x86-64? On i386, the "... & 1" is consistently used across all of 
> these,
> but on x86-64 only the __pte/pte_val pair does so.
We only ever test '& 1' in pte_val(). In higher-level *_val functions 
we need not limit the test to bit 0 (because an unused page directory 
entry is always 0 in Linux) and also we *cannot* limit the test to bit 
0 (at least for pmd_val()) because Xen will silently temporarily remove 
_PAGE_PRESENT from page directory entries as part of the 
writable-pagetable implementation.
It is certainly arguable that the __pgd/__pud/__pmd macros should do 
the same test as the corresponding *_val function, for symmetry, but 
neither xen/i386 nor xen/x86_64 is symmetric right now (at least in 
xen-unstable.hg).
 -- Keir


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