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

To: Jeremy Fitzhardinge <jeremy@xxxxxxxx>
Subject: [Xen-devel] Re: [PATCH 1 of 7] x86: add _PAGE_IOMAP pte flag for IO mappings
From: Avi Kivity <avi@xxxxxxxxxxxx>
Date: Tue, 09 Sep 2008 17:56:35 +0300
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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Jeremy Fitzhardinge wrote:
Avi Kivity wrote:
Jeremy Fitzhardinge wrote:
Use one of the software-defined PTE bits to indicate that a mapping is
intended for an IO address.  On native hardware this is irrelevent,
since a physical address is a physical address.  But in a virtual
environment, physical addresses are also virtualized, so there needs
to be some way to distinguish between pseudo-physical addresses and
actual hardware addresses; _PAGE_IOMAP indicates this intent.

By default, __supported_pte_mask masks out _PAGE_IOMAP, so it doesn't
even appear in the final pagetable.
Could PTE_SPECIAL, added for get_user_pages_really_fast(), be reused
for this?

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.

Is _PAGE_IOMAP at all useful for device passthrough in kvm?

No. We don't care if a page is an I/O page or RAM (other than refcounting correctness and page attributes, which are handled by other means).

error compiling committee.c: too many arguments to function

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