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

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Date:	Thu, 6 Jan 2011 19:50:26 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	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>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	Keir Fraser <keir@....org>
Subject: Re: [PATCH 6/8] xen/debug: WARN_ON when 1-1 but no _PAGE_IOMAP flag
 set.

On Tue, 4 Jan 2011, Ian Campbell wrote:
> On Tue, 2011-01-04 at 18:46 +0000, Konrad Rzeszutek Wilk wrote: 
> > On Tue, Jan 04, 2011 at 05:24:38PM +0000, Ian Campbell wrote:
> > > On Thu, 2010-12-30 at 19:48 +0000, Konrad Rzeszutek Wilk wrote:
> > > > Only enabled if XEN_DEBUG_FS is enabled.
> > > 
> > > Bit of an odd configuration option to use. Perhaps co-opt
> > > CONFIG_PARAVIRT_DEBUG instead?
> > > 
> > > Or maybe we need a new XEN_DEBUG option? or just make it a developer
> > > only EXTRA_CFLAGS +=-DDEBUG thing?
> > 
> > I think the 'XEN_DEBUG' works better.
> > > 
> > > Is this only temporary until the need for _PAGE_IOMAP is removed anyway?
> > 
> > I was thinking to leave it as a way to weed out bugs, but I could as well
> > just leave it in my "debug" branch and not propose it upstream.
> > 
> > I am not sure how to remove the _PAGE_IOMAP fully. We need some _flag_ to
> > signal 'xen_pte_val' that the PTE should not be looked up in the M2P.
> > 
> > Otherwise, for identity mappings, the value is 0xfffff.. (or 0x55555.. sometimes)
> > and the PTE ends up being messed up. Instead there is a check to see if
> > _PAGE_IOMAP is part of the PTE, and if so, no M2P lookup is done.
> > 
> > I guess we could do the M2P irregardless and just see if it is 0xfffff... and
> > if so just return the PTE without any changes.
> 
> 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?
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