linux-kernel - Re: [PATCH 6/8] xen/debug: WARN_ON when 1-1 but no _PAGE

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Date:	Thu, 06 Jan 2011 20:17:38 +0000
From:	Keir Fraser <keir@....org>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Ian Campbell <Ian.Campbell@...citrix.com>
CC:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	"hpa@...or.com" <hpa@...or.com>, Jan Beulich <JBeulich@...ell.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	Konrad Rzeszutek Wilk <konrad@...nel.org>
Subject: Re: [PATCH 6/8] xen/debug: WARN_ON when 1-1 but no _PAGE_IOMAP flag
 set.

On 06/01/2011 19:50, "Stefano Stabellini" <stefano.stabellini@...citrix.com>
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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