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Re: [Xen-devel] live migration question

  • To: Keir Fraser <keir@xxxxxxx>
  • From: Juergen Gross <juergen.gross@xxxxxxxxxxxxxx>
  • Date: Tue, 31 Jan 2012 14:58:55 +0100
  • Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, Jan Beulich <JBeulich@xxxxxxxx>
  • Delivery-date: Tue, 31 Jan 2012 13:59:17 +0000
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On 01/31/2012 01:56 PM, Keir Fraser wrote:
On 31/01/2012 12:42, "Juergen Gross"<juergen.gross@xxxxxxxxxxxxxx>  wrote:

I've made an additional test using xm save/xm restore to see if the same
problem shows up. It does NOT. Mapping succeeds at once while restoring
memory is still running. I always thought xm restore and live migration on
the target machine are more or less the same. This seems not to be true.
Okay, here are my results (so far):

do_mmu_update() calls mod_l1_entry() which fails with -EINVAL due to an
invalid mfn and p2m-type == 4 returned by:

mfn_x(gfn_to_mfn(pg_dom, l1e_get_pfn(nl1e),&p2mt))

I still don't see why xc_restore is able to do the mapping while my daemon is
not. And I can't find any difference between a domain creation due to
xm restore and a live migration.
Memory pages are populated as they appear in the migration data stream.
Perhaps you are trying to map a guest page that has simply not yet been

As far as I can tell, memory is transferred from low to high mfns. The first
iteration takes about 12 minutes in my test case. Why is the mapping possible
only after the last iteration has finished? The mfn I try to map is in the
first 16 MB of the domain, so it should arrive in the first second!

Bear in mind that IOCTL_PRIVCMD_MMAPBATCH_V2 will note, but proceed past,
failed individual mappings. While IOCTL_PRIVCMD_MMAP, for example, will fail
the entire ioctl() in that situation. There's a reason that xc_restore uses
the former ioctl!

I understand that. But what is the difference between xm restore and live
migration? Somehow the memory seems to be treated different.


Juergen Gross                 Principal Developer Operating Systems
PDG ES&S SWE OS6                       Telephone: +49 (0) 89 3222 2967
Fujitsu Technology Solutions              e-mail: juergen.gross@xxxxxxxxxxxxxx
Domagkstr. 28                           Internet: ts.fujitsu.com
D-80807 Muenchen                 Company details: ts.fujitsu.com/imprint.html

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