Re: [Xen-devel] xm pause <domain>

[Mark Williamson <Mark.Williamson@xxxxxxxxxxxx>]

> I would like to know how does the pause functionality works with xm. I am
> able to use it but not sure what exactly it does. The documentation
> mentions it but doesn't really say what does it do or what is it used for.
> My assumption is it stops the CPU cycles provided to the domain and so the
> domain kind of "freezes".

Yes, it hypercalls into Xen and causes the Xen CPU scheduler to no longer give 
any CPU time to the domain.

> Also, once the domain is paused, is it possible to do some analysis on the
> state... like what processes are running (which were froze now), what files
> were open etc. from the hypervisor using the existing tools.

Not as such.  Kip wrote some patches allowing GDB to analyse the state of a 
domain but this would be debugger level functionality - to do what you 
describe directly requires a bit more OS-specific knowledge.

> Also, if such 
> tools don't exist, how difficult or easy would it be to develop such tools
> and which parts of the code would have to be modified.

The Linux Kernel Crash Dump project (http://lkcd.sf.net) includes 
Linux-specific analysis tools that (given an LKCD-formatted dump file) can 
inspect what processes were running when a dump was taken, etc.  It would be 
nice if we could dump a domain in LKCD format and thus allow these tools to 
be leveraged.

> Where does the code 
> for pause exist?

IIRC, the code in Xen is in xen/common/dom0_ops.c and xen/common/schedule.c.  
The code for calling this from user space is in the Xc libs, which are called 
by Xend on receiving a request from the xm tool.

> I would really appreciate any help in this regard.

HTH,
Mark

>
> Thanks,
>
> Chotu
>
>
>
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