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Date: Fri, 23 Jan 2009 08:14:06 +0000
From: "Jan Beulich" <jbeulich@novell.com>
To: "Ingo Molnar" <mingo@elte.hu>, "Jeremy Fitzhardinge" <jeremy@goop.org>
Cc: "Xen-devel" <xen-devel@lists.xensource.com>,
       "Linux Kernel Mailing List" <linux-kernel@vger.kernel.org>,
       "Nick Piggin" <nickpiggin@yahoo.com.au>
Subject: Re: [Xen-devel] [PATCH 2/2] x86: add pte_set_flags/clear_flags
	 for pteflag manipulation
References: <4978F216.5040803@goop.org>
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>>> Jeremy Fitzhardinge <jeremy@goop.org> 22.01.09 23:24 >>>
>+static inline pte_t pte_set_flags(pte_t pte, pteval_t set)
>+{
>+	pteval_t v = native_pte_val(pte);
>+
>+	return native_make_pte(v | set);
>+}
>+
>+static inline pte_t pte_clear_flags(pte_t pte, pteval_t clear)
>+{
>+	pteval_t v = native_pte_val(pte);
>+
>+	return native_make_pte(v & ~clear);
>+}

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.

Jan