live migration question

[Juergen Gross <juergen.gross@ts.fujitsu.com>]

Hi,

I've got a question regarding live migration:

We have enabled our mainframe OS BS2000 to run as pv-hvm domain on Xen. Live
migration is working rather fine, but we have timing problems with large
memory (e.g. 128 GB). The reason for the timing problem is our own backend
driver for the BS2000 devices which requires to map all the domain memory to
avoid too many changes in BS2000. The mapping occurs at reconnection to the
backend driver and has to be synchronous. For a 128 GB domain this leads to
a 16 second stall of the domain when it is resuming on the target machine.

To avoid this stall I tried to start a little daemon on the target machine
and watch for a new BS2000 domain to show up due to live migration. I wanted
to map the domain memory as soon as the needed mapping information located in
a fixed guest mfn was transferred. Discovery of the new domain works as
expected, but I'm not capable doing any memory mapping until the restore of
the domain is finished. The mapping ioctl using IOCTL_PRIVCMD_MMAP returns
EINVAL until xc_restore is finished (more or less).

Why can xc_restore do the mapping while I can't? I know xc_restore is using
IOCTL_PRIVCMD_MMAPBATCH_V2, but I can't see a difference which should matter
between those two, as both are using the same hypercall to update the dom0
page tables.

Xen version is 4.0.2, linux kernel is 2.6.32 (both SLES11 SP1), the machine
is a x86_64 INTEL based one.


Juergen

-- 
Juergen Gross                 Principal Developer Operating Systems
PDG ES&S SWE OS6                       Telephone: +49 (0) 89 3222 2967
Fujitsu Technology Solutions              e-mail: juergen.gross@ts.fujitsu.com
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