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[Xen-devel] Re: [PATCH 6/8] xen/debug: WARN_ON when 1-1 but no _PAGE_IOM

To: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH 6/8] xen/debug: WARN_ON when 1-1 but no _PAGE_IOMAP flag set.
From: Keir Fraser <keir@xxxxxxx>
Date: Thu, 06 Jan 2011 20:17:38 +0000
Cc: Jeremy Fitzhardinge <jeremy@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>, "hpa@xxxxxxxxx" <hpa@xxxxxxxxx>
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On 06/01/2011 19:50, "Stefano Stabellini" <stefano.stabellini@xxxxxxxxxxxxx>

>> Perhaps this ties in with the m2p overlay which Stefano+Jeremy have been
>> working on to deal with granted foreign pages? I/O pages are a bit like
>> foreign memory (if you squint enough)...
> In theory the m2p overlay could be used for this purpose but in practice
> the current m2p overlay API needs a struct page, also it might end up
> stressing the hashtable too much.
> Besides I think Konrad's solution might be simpler: if the m2p returns
> one of the two special values we just return mfn from pte_mfn_to_pfn.
> Keir, could you confirm that the m2p entries of DOM_IO pages are always
> 0xffffff or 0x55555?

Always 0x55...55 (for m2p entries that exist), else page fault on access to
the non-existent m2p entry (m2p entries only guaranteed to exist for ram).
Perhaps the 0xff...ff values come from Linux's own fixup code handling a
faulting read access of the m2p array? If so you could return 0x55...55
instead and avoid checking for 0xff...ff. I really don't know how you could
get 0xff...ff for non-RAM pages from Xen itself.

 -- Keir

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