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

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Date:	Mon, 26 Jan 2009 07:51:24 +0000
From:	"Jan Beulich" <jbeulich@...ell.com>
To:	"Jeremy Fitzhardinge" <jeremy@...p.org>
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	 forpteflag manipulation

>>> Jeremy Fitzhardinge <jeremy@...p.org> 23.01.09 22:05 >>>
>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).

Whether a pte is live doesn't matter here: If you change P on Xen, the
frame number representation *must* change from/to PFN to/from MFN.
In no case (other than iomem pages) is it allowed to flip just this bit.

Jan

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