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xen-devel
[Xen-devel] Re: [PATCH 6/8] xen/debug: WARN_ON when 1-1 but no _PAGE_IOM
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To: |
Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx> |
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Subject: |
[Xen-devel] Re: [PATCH 6/8] xen/debug: WARN_ON when 1-1 but no _PAGE_IOMAP flag set. |
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From: |
Keir Fraser <keir@xxxxxxx> |
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Date: |
Thu, 06 Jan 2011 20:17:38 +0000 |
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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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Thu, 06 Jan 2011 12:19:07 -0800 |
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[PATCH 6/8] xen/debug: WARN_ON when 1-1 but no _PAGE_IOMAP flag set. |
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On 06/01/2011 19:50, "Stefano Stabellini" <stefano.stabellini@xxxxxxxxxxxxx>
wrote:
>> 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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