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 22:17:06 +0000
From:	Keir Fraser <keir@....org>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	Ian Campbell <Ian.Campbell@...citrix.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 21:59, "Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com> wrote:

>> 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.
> 
> The non-RAM pages are assinged to a DOMID_IO (arch_init_memory), for example:
> 
>  298     /* First 1MB of RAM is historically marked as I/O. */
>  299     for ( i = 0; i < 0x100; i++ )
>  300         share_xen_page_with_guest(mfn_to_page(i), dom_io,
> XENSHARE_writable);
> 
> and share_xen_page.. sets that page to INVALID_M2P_ENTRY.
> 
> But I could also be reading the code wrongly?

You're right, I missed that.

 -- Keir


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