Re: Xen 4.7 crash

[Aaron Cornelius <aaron.cornelius@dornerworks.com>]

On 6/1/2016 6:35 PM, Julien Grall wrote:
> Hello Aaron,
>
> On 01/06/2016 20:54, Aaron Cornelius wrote:
<snip>
>> I'm not 100% sure, from the "VMID pool exhausted" message it would
>> appear that the p2m_init() function failed to allocate a VM ID, which
>> caused domain creation to fail, and the NULL pointer dereference when
>> trying to clean up the not-fully-created domain.
>>
>> However, since I only have 1 domain active at a time, I'm not sure why
>> I should run out of VM IDs.
>
> arch_domain_destroy (and p2m_teardown) is only called when all the
> reference on the given domain are released.
>
> It may take a while to release all the resources. So if you launch the
> domain as the same time as you destroy the previous guest. You will have
> more than 1 domain active.
>
> Can you detail how you create/destroy guest?
>

This is with a custom application, we use the libxl APIs to interact 
with Xen.  Domains are created using the libxl_domain_create_new() 
function, and domains are destroyed using the libxl_domain_destroy() 
function.

The test in this case creates a domain, waits a minute, then 
deletes/creates the next domain, waits a minute, and so on.  So I 
wouldn't be surprised to see the VMID occasionally indicate there are 2 
active domains since there could be one being created and one being 
destroyed in a very short time.  However, I wouldn't expect to ever have 
256 domains.

The CubieTruck only has 2GB of RAM, I allocate 512MB for dom0 which 
means that only 48 of the the Mirage domains (with 32MB of RAM) would 
work at the same time anyway.  Which doesn't account for the various 
inter-domain resources or the RAM used by Xen itself.

If the p2m_teardown() function checked for NULL it would prevent the 
crash, but I suspect Xen would be just as broken since all of my 
resources have leaked away.  More broken in fact, since if the board 
reboots at least the applications will restart and domains can be recreated.

It certainly appears that some resources are leaking when domains are 
deleted (possibly only on the ARM or ARM32 platforms).  We will try to 
add some debug prints and see if we can discover exactly what is going on.

- Aaron Cornelius


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