Re: [Qemu-devel] Runtime-modified DIMMs and live migration issue

[Andrey Korolyov]

> Answering back to myself - I made a wrong statement before, the
> physical mapping *are* different with different cases, of course!
> Therefore, the issue looks much simpler and I`d have a patch over a
> couple of days if nobody fix this earlier.
>

... and another (possibly last) update. This is not a memory
misplacement but a quirky race - if no internal workload applied to
the virtual machine during migration, no harm is done - after its
completion the VM passes all imaginable tests as well. If no device
I/O is involved (perf bench numa for 1000s), guest is not falling with
guts out as with disk-test case, just crashing a process instead:

[  784.613032] thread 0/0[2750]: segfault at 0 ip           (null) sp
00007fda4ea6a940 error 14 in perf_3.16[400000+144000]

I think we are facing a very interesting memory access race during a
live migration but there are no visible reasons for it to be bound
only for a runtime-plugged memory case. All possible cases where
either userspace or kernel driver are involved showing null as an
instruction pointer for trace, may be this can be a hint for someone.