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

To: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] Re: [PATCH 1 of 7] x86: add _PAGE_IOMAP pte flag for IO mappings
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Tue, 09 Sep 2008 08:48:06 -0700
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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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.

At present we can't set _PAGE_SPECIAL on kernel mappings because we
assume its usermode only, and its shared with _PAGE_CPA_TEST which is
kernel-only.  Most _PAGE_IOMAP mappings are in kernel space, but I
wouldn't discount user mappings if we end up allowing direct access to
passthrough devices.


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