linux-kernel - Re: [Xen-devel] [PATCH 2/2] x86: add pte_set_flags/clear

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Message-ID: <497A3117.2070809@goop.org>
Date:	Fri, 23 Jan 2009 13:05:27 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Jan Beulich <jbeulich@...ell.com>
CC:	Ingo Molnar <mingo@...e.hu>,
	Xen-devel <xen-devel@...ts.xensource.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <nickpiggin@...oo.com.au>
Subject: Re: [Xen-devel] [PATCH 2/2] x86: add pte_set_flags/clear_flags	 for
 pteflag manipulation

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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