Re: [PATCH 6/8] xen/debug: WARN_ON when 1-1 but no _PAGE_IOMAP flagset.

From: Keir Fraser
Date: Thu Jan 06 2011 - 15:18:10 EST

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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