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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>
Subject: Re: [Xen-devel] Re: [PATCH 1 of 7] x86: add _PAGE_IOMAP pte flag for IO mappings
From: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Date: Wed, 10 Sep 2008 09:38:36 -0700
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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Avi Kivity wrote:
> Actually it's more of a "no struct page" flag, which implies no
> refcounting.

Hm, is that actually true enough to define it?  Could we rename it
something like _PAGE_NOSTRUCTPAGE or something a bit more specific than

>   And not having a struct page should correspond well to a pte not
> requiring pfn->mfn conversion and being an I/O page.

But _PAGE_SPECIAL is only set in a few places.  It's not set in ioremap
mappings and so on.  Should it be?

There's also the hiccup that it gets set in a pte with pte_mkspecial() -
but at that point its too late because you've already constructed the
pte and done the pfn->mfn conversion.  _PAGE_IOMAP can only be set when
you initially construct the pte out of a frame number and a pgprot.


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