linux-kernel - Re: [PATCH 4/9] Vmi fix highpte

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Message-ID: <45E79529.9000305@goop.org>
Date:	Thu, 01 Mar 2007 19:08:25 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Zachary Amsden <zach@...are.com>
CC:	Andi Kleen <ak@....de>, Linus Torvalds <torvalds@...l.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Chris Wright <chrisw@...s-sol.org>,
	Dan Hecht <dhecht@...are.com>, Dan Arai <arai@...are.com>,
	Andrew Morton <akpm@...l.org>,
	Virtualization Mailing List <virtualization@...ts.osdl.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/9] Vmi fix highpte

Zachary Amsden wrote:
> Provide a PT map hook for HIGHPTE kernels to designate where they are mapping
> page tables.  This information is required so the physical address of PTE
> updates can be determined; otherwise, the mm layer would have to carry the
> physical address all the way to each PTE modification callsite, which is
> even more hideous that the macros required to provide the proper hooks.
>
> So lets not mess up arch neutral code to achieve this, but keep the horror
> in an #ifdef HIGHPTE in include/asm-i386/pgtable.h.  I had to use macros
> here because some types are not yet defined in all the include paths for
> this header.
>
> This patch is absolutely required for HIGHPTE kernels to operate properly
> with VMI.
>   

Hm, I don't think this interface will work for Xen.  In Xen, whenever a
pagetable page gets mapped, it must be mapped RO.  map_pt_hook gets
called after the mapping has already been created, so its too late for Xen.

I was planning on adding kmap_atomic_pte() for use in pte_offset_map*(),
which would be wired through to paravirt_ops to allow Xen to make this a
RO mapping.  Would this be sufficient for you to do your vmi thing?

    J
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