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Re: [Xen-users] 101th domU fails to start with "SETVCPUCONTEXT failed"

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Subject: Re: [Xen-users] 101th domU fails to start with "SETVCPUCONTEXT failed"
From: Lennert Van Alboom <lennert.vanalboom@xxxxxxxxxx>
Date: Mon, 11 Jan 2010 14:08:46 +0100
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On Sun, Jan 10, 2010 at 12:30:12AM +0100, storm66@xxxxxxxxxxxxxxxx wrote:
Le samedi 09 janvier 2010 à 21:57 +0100, Lennert Van Alboom a écrit :
Hello there,

We (a small hosting community) are running a steadily growing number of Xen domUs on a quad dualcore Xeon server with 64GB ram. We've got 100 running domUs at the moment. Trying to create a new one results in this error:

Error: (1, 'Internal error', 'launch_vm: SETVCPUCONTEXT failed (rc=-1)\n')

If I shut down another domain, I can create one again, but the limit
seems to be 100 domUs. Each domU has one network interface and one or two disk devices (raid-1
md device consisting of two iscsi luns).

Version of Xen is rather old: 3.2.1-amd64 (Debian).
Can anyone give me hint as to what causes this behaviour? The host
machine should be able to cope with the amount of domUs easily. I've got
a feeling I'm bumping into an obscure sourcecode parameter somewhere.

Thanks in advance,



In my thought Xen is heavily using loop devices, I had some problems
some years ago with a system running XEN 3.0 with a kernel where the
default loop max number was too low. I didn't get that problem with newer versions.



Thanks for your reply. We're not using loopbacks though - only physical
devices (raid1 md).
I've been tracking this problem through the xen source, and found this:
The launch_vm code is located in
; the function that is causing the return code of -1 is do_domctl.

I'm unsure as to where do_domctl is really located - I've found two
. Neither give me much of a clue. In case of the first definition, the returncode is given by ret = do_xen_hypercall(xc_handle, &hypercall) which leads me further to ioctl(xc_handle, cmd, data); and after that I'm stumped. In the second case, the -1 comes *either* from IS_PRIV(current->domain)
failing, causing an EPERM error. This sounds unlikely. The other place
where a -1 could be given is ret = xsm_setvcpucontext(d);
which translates back to
and there I'm stuck again. I'm totally not capable of deciphering
xsm_ops and its meanings.

Does anyone with some insight in the innards of Xen have a clue on what
might be causing our issues?

Kind regards,


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