Re: [Xen-devel] Xen 4.2.1 live migration with qemu device model

[Alex Bligh <alex@xxxxxxxxxxx>]

Ian,

(two messages in one)

> > In 4.2.0, adding this patch and attempting a live migrate using
> > the qemu device model (using xl) produces a seg fault due to
> > unitialised variables.
> Really using xl? because the stack trace suggests otherwise.
Sorry, libxl.

> > Should I expect live-migrate of qemu-dm vms to work under 4.2.1?
> Do you mean VMs using the upstream "qemu-xen" device model, as opposed
> to the default "qemu-xen-traditional" model?
Yes, using upstream qemu-xen.

> Migration of HVM guests in 4.2.x is only supported with the
> qemu-xen-traditional device model and AFAIK there is no plan to backport
> this support to 4.2.
Ah. What would be involved in a backport? We use HVM guests under 4.2 and
need qemu-xen for various reasons.

> It still shouldn't crash though. I'm not sure how it got this far since
> libxl on 4.2 explicitly checks the DM version before attempting to
> migrate and will refuse to even try with a qemu-xen DM.
We had removed the check (per the pointer to the mail message I sent)
for the qemu-xen model.

> libxl__xs_read_checked will always either initialise the variable
> (perhaps to NULL) or return an error. On both callsites we check for
> error and "goto out".
It returns NULL if the error is not ENOENT I think.

> I think the crash is because the code uses got_ret without checking if
> it was NULL, which can happen if the path is not present. Ian (J) does
> that make sense as something which is allowed to happen?
gdb showed one pointer was NULL and the other pointed to some rubbish
(the latter is confusing).

Alex



> Ian.
>
> > If so, should the patch (or a modification thereof) to remove
> > the check from libxl_domain_suspend be applied to 4.2.1-testing
> > or is there more to do?
> >
> > I am very happy to commit test resources to this.
> >
> > Program received signal SIGSEGV, Segmentation fault.
> > [Switching to Thread 0x7fffeffff700 (LWP 5995)]
> > 0x00007ffff5a0862a in ?? () from /lib/x86_64-linux-gnu/libc.so.6
> > (gdb) bt
> > # 0  0x00007ffff5a0862a in ?? () from /lib/x86_64-linux-gnu/libc.so.6
> > # 1  0x00007ffff6d4e970 in libxl__domain_suspend_common_switch_qemu_logdirty 
> > (domid=<optimized out>, enable=<optimized out>, user=0x7ffff00024e8) at 
> > libxl_dom.c:728
> > # 2  0x00007ffff6d5c1ae in libxl__srm_callout_received_save (msg=0x7fffefffe41a " 
> > error", len=<optimized out>, user=0x7ffff00024e8) at _libxl_save_msgs_callout.c:162
> > # 3  0x00007ffff6d5b736 in helper_stdout_readable (egc=0x7fffefffe5a0, ev=0x7ffff0002560, 
> > fd=38, events=<optimized out>, revents=<optimized out>) at 
> > libxl_save_callout.c:283
> > # 4  0x00007ffff6d601f1 in afterpoll_internal (egc=0x7fffefffe5a0, 
> > poller=0x7ffff00028c0, nfds=4, fds=0x7ffff00048b0, now=...) at libxl_event.c:948
> > # 5  0x00007ffff6d604db in eventloop_iteration (egc=0x7fffefffe5a0, 
> > poller=0x7ffff00028c0) at libxl_event.c:1368
> > # 6  0x00007ffff6d616b3 in libxl__ao_inprogress (ao=0x7ffff0001d40, file=<optimized out>, 
> > line=<optimized out>, func=<optimized out>) at libxl_event.c:1614
> > # 7  0x00007ffff6d3ab75 in libxl_domain_suspend (ctx=<optimized out>, domid=1, fd=10, 
> > flags=<optimized out>, ao_how=<optimized out>) at libxl.c:796
> > # 8  0x000000000043677e in migrate_domain_send (ctx=0x7ffff0008860, domid=1, 
> > fd=10) at hypervisor/xen_libxl.c:587
> > # 9  0x000000000043698a in live_migrate_send (hyperconn=0x7ffff0001c70, 
> > server=0x7ffff0001cb0, node_ip=0x7ffff00041e0 "10.157.128.20", fd=10) at 
> > hypervisor/xen_libxl.c:647
> > # 10 0x0000000000422a70 in migrate_server_action (request=0x7ffff0002980) at 
> > action/node_action.c:1287
> > # 11 0x00000000004240c1 in runAction (socket_fd=8) at action/handleaction.c:138
> > # 12 0x00000000004179bd in runcomm (socket=0x8) at xvpagent.c:253
> > # 13 0x0000000000427502 in trackedthread_run (arg=0x66df20) at util/util.c:179
> > # 14 0x00007ffff5c9ce9a in start_thread () from 
> > /lib/x86_64-linux-gnu/libpthread.so.0
> > # 15 0x00007ffff59ca4bd in clone () from /lib/x86_64-linux-gnu/libc.so.6
> > # 16 0x0000000000000000 in ?? ()
>
>
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--
Alex Bligh

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