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xen-devel
[Xen-devel] Re: [PATCH 6/8] xen/debug: WARN_ON when 1-1 but no _PAGE_IOM
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To: |
Keir Fraser <keir@xxxxxxx> |
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Subject: |
[Xen-devel] Re: [PATCH 6/8] xen/debug: WARN_ON when 1-1 but no _PAGE_IOMAP flag set. |
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From: |
Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> |
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Date: |
Thu, 6 Jan 2011 16:59:52 -0500 |
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Cc: |
Jeremy Fitzhardinge <jeremy@xxxxxxxx>, "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>, "linux-kernel@xxxxxxxxxxxxxxx" <linux-kernel@xxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxxxx>, Ian Campbell <Ian.Campbell@xxxxxxxxxxxxx>, Konrad Rzeszutek Wilk <konrad@xxxxxxxxxx>, "hpa@xxxxxxxxx" <hpa@xxxxxxxxx> |
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Thu, 06 Jan 2011 14:02:50 -0800 |
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<C94BD3E2.11441%keir@xxxxxxx> |
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<alpine.DEB.2.00.1101061935070.2390@kaball-desktop> <C94BD3E2.11441%keir@xxxxxxx> |
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Mutt/1.5.20 (2009-06-14) |
On Thu, Jan 06, 2011 at 08:17:38PM +0000, Keir Fraser wrote:
> On 06/01/2011 19:50, "Stefano Stabellini" <stefano.stabellini@xxxxxxxxxxxxx>
> 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.
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?
>
> -- Keir
>
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