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Date: Thu, 06 Jan 2011 22:17:06 +0000
Subject: Re: [PATCH 6/8] xen/debug: WARN_ON when 1-1 but no _PAGE_IOMAP flag
 set.
From: Keir Fraser <keir@xen.org>
To: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
CC: Stefano Stabellini <stefano.stabellini@eu.citrix.com>,
        Ian Campbell <Ian.Campbell@eu.citrix.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Jeremy Fitzhardinge <jeremy@goop.org>, "hpa@zytor.com" <hpa@zytor.com>,
        Jan Beulich <JBeulich@novell.com>,
        "xen-devel@lists.xensource.com" <xen-devel@lists.xensource.com>,
        Konrad Rzeszutek Wilk <konrad@kernel.org>
Message-ID: <C94BEFE2.11452%keir@xen.org>
Thread-Topic: [PATCH 6/8] xen/debug: WARN_ON when 1-1 but no _PAGE_IOMAP flag
 set.
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On 06/01/2011 21:59, "Konrad Rzeszutek Wilk" <konrad.wilk@oracle.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