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Re: [Xen-devel] Re: [PATCH 1 of 7] x86: add _PAGE_IOMAP pte flag for IO

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH 1 of 7] x86: add _PAGE_IOMAP pte flag for IO mappings
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Tue, 09 Sep 2008 17:05:00 +0100
Cc: Xen Devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, Avi Kivity <avi@xxxxxxxxxxxx>, Andi Kleen <andi@xxxxxxxxxxxxxx>, "H. Peter Anvin" <hpa@xxxxxxxxx>, Ingo Molnar <mingo@xxxxxxx>
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Thread-topic: [Xen-devel] Re: [PATCH 1 of 7] x86: add _PAGE_IOMAP pte flag for IO mappings
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On 9/9/08 16:48, "Jeremy Fitzhardinge" <jeremy@xxxxxxxx> wrote:

> Keir Fraser wrote:
>> That's basically what our _PAGE_IO flag (in our old Linux patchset) means.
>> We use it to cause pte_pfn() to return an invalid pfn and hence avoid
>> reference counting that way. Since kernel mappings are never reference
>> counted (I think?) perhaps we could use _PAGE_SPECIAL even if it is
>> restricted to use on user mappings.
> Well, _PAGE_IOMAP's most important semantic from Xen's perspective is
> that the frame number is considered to already be an MFN and so isn't
> converted.  It may be that _PAGE_SPECIAL is also useful for its "no
> refcount" properties, but we could set both in that case.

Oh, yes, our _PAGE_IO has the same meaning.

 -- Keir

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