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Re: [Xen-devel] [PATCH 2/2] x86: add pte_set_flags/clear_flags for ptef

To: Jan Beulich <jbeulich@xxxxxxxxxx>
Subject: Re: [Xen-devel] [PATCH 2/2] x86: add pte_set_flags/clear_flags for pteflag manipulation
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Fri, 23 Jan 2009 13:05:27 -0800
Cc: Nick Piggin <nickpiggin@xxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>
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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.

   J

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