Re: [Xen-devel] Problems using xl migrate

[M A Young <m.a.young@xxxxxxxxxxxx>]

On Sat, 22 Nov 2014, M A Young wrote:

> While investigating a bug reported on Red Hat Bugzilla
> https://bugzilla.redhat.com/show_bug.cgi?id=1166461
> I discovered the following
>
> xl migrate --debug domid localhost does indeed fail for Xen 4.4 pv (the bug 
> report is for Xen 4.3 hvm ) when xl migrate domid localhost works. There are 
> actually two issues here
> * the segfault in libxl-save-helper --restore-domain (as reported in the bug 
> above) occurs if the guest memory is 1024M (on my 4G box) and is presumably 
> because the allocated memory eventually runs out
I have found a bit more out about this. The segfault at at line 1378 of 
tools/libxc/xc_domain_restore.c which is
                DPRINTF("************** pfn=%lx type=%lx gotcs=%08lx "
                        "actualcs=%08lx\n", pfn, pagebuf->pfn_types[pfn],
                        csum_page(region_base + (i + curbatch)*PAGE_SIZE),
                        csum_page(buf));
and is because pfn in pagebuf->pfn_types[pfn] is beyond the end of the 
array. This occurs in the verification phase.
> * the segfault doesn't occur if the guest memory is 128M, but the migration 
> still fails. The first attached file contains the log from a run with xl -v 
> migrate --debug domid localhost (with mfn and duplicated lines stripped out 
> to make the size manageable).
The difference actually seems to be down to how active the VM is rather 
than the memory size (my small memory test system was doing very little, 
my larger system was a full OS install). In the non-segfault case the 
problem was the printf and printf_info commands in the create_domain() 
routine in tools/libxl/xl_cmdimpl.c . As xl migrate uses stdout to pass 
status messages back from the restoring dom0, these commands cause an 
unexpected message. If you move them onto stderr then the migration 
completes in the non-segfault case.
        Michael Young

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