Re: [Xen-devel] [PATCH 2/2] x86: add pte_set_flags/clear_flags forpteflag manipulation

From: Jeremy Fitzhardinge
Date: Fri Jan 23 2009 - 16:05:40 EST

Jan Beulich wrote:
I think a comment (or event a BUG_ON()) should be added here to make
clear that this absolutely must not be used to toggle the present bit. I
even view toggling _PAGE_PSE as dangerous this way.

And alternative would be to make these macros and #undef them (or keep
them inline functions but add destructive #define-s) after all their users.

I don't see any particular problem with changing PSE or even Present with these functions; they don't operate on live in-memory ptes, so its not like they could ever be used to modify a pte unless followed with some kind of set_pte operation. It is unwise to change any pte flag without knowing what you're doing (though P or PSE would probably have less subtle effects than some of the others).

But it probably wouldn't hurt to have a __ prefix to indicate they're "internal" and not for general use.

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