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

To: Avi Kivity <avi@xxxxxxxxxxxx>, 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 16:29:04 +0100
Cc: Ingo Molnar <mingo@xxxxxxx>, Xen Devel <xen-devel@xxxxxxxxxxxxxxxxxxx>, Andi Kleen <andi@xxxxxxxxxxxxxx>, linux-kernel@xxxxxxxxxxxxxxx, "H. Peter Anvin" <hpa@xxxxxxxxx>
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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 15:56, "Avi Kivity" <avi@xxxxxxxxxxxx> wrote:

>> I'm not sure; I still don't really understand how _PAGE_SPECIAL gets
>> used, other than being user-mode mapping only.  But in principle,
>> _PAGE_IOMAP could be set on both kernel and user mappings (if you direct
>> map a device into a process address space), so I think they would
>> conflict then?
> It's a "don't refcount me" flag, which is not sematically the same as
> I/O, but may be close enough.

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.

 -- Keir

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