Re: [Qemu-devel] [PATCH 00/13] instrument: Add basic event instrumentation

["Daniel P. Berrange" <berrange@redhat.com>]

On Thu, Jul 27, 2017 at 04:33:01PM +0100, Peter Maydell wrote:
> On 27 July 2017 at 16:21, Daniel P. Berrange <berrange@redhat.com> wrote:
> > On Thu, Jul 27, 2017 at 11:54:29AM +0100, Peter Maydell wrote:
> >> That said, yes, I was going to ask if we could do this via
> >> leveraging the tracepoint infrastructure and whatever scripting
> >> facilities it provides. Are there any good worked examples of
> >> this sort of thing? Can you do it as an ordinary non-root user?
> >
> > Do you have a particular thing you'd like to see an example of ?
> >
> > To dynamically probe a function which doesn't have a tracepoint
> > defined you can do:
> >
> > probe process("/usr/bin/qemu-x86_64").function("helper_syscall") {
> >   printf("syscall stasrt\n")
> > }
> >
> > but getting access to the function args is not as easy as with
> > pre-defined tracepoints.
> 
> How do I go about actually running that script? What I
> have in mind by "worked example" is something like a blog
> post that says "ok, here's a problem, we want to find out
> what QEMU is doing in situation X, here's how you do this
> with $TRACING_THINGY" and generally steps you through how
> it works assuming you know nothing at all about whatever
> the tracing facility you're using is.

Ok, so something like this example that I wrote for libvirt a
while back then

  https://www.berrange.com/posts/2011/11/30/watching-the-libvirt-rpc-protocol-using-systemtap/


> > You can't typically run this as root,
> 
> Do you mean "non-root" ?

Sigh, yes, of course.

> > however, I don't think that's a
> > huge issue, because most QEMU deployments are not running as your own
> > user account anyway, so you can't directly interact with them no
> > matter what.
> 
> It is important, because almost all uses of TCG QEMU are
> running it from the command line as non-root normal users,
> especially if they're trying to debug what's going on with a
> guest binary. So any tracing solution for this kind of usecase
> must work without requiring root access, I think.

None of the Linux integrated tracing tools allow direct non-root access
afaik. systemtap has ability to launch probes as non-root, via a privileged
daemon, but it is restricted to probe scripts that the administrator has
pre-defined.

That pretty much leaves re-building QEMU, LD_PRELOADS, or something
ptrace(), or qemu's built-in simpletrace feature, as the remaining
options.  We have a scripts/simpletrace.py that lets you load a
trace file into python and process it, but as written that's aimed
as post-processing a tracefile you've previously collected.

It would be desirable to write a more advanced simpletrace python
module that could collect & process the trace data live, and also
interact with the qemu monitor to change what events are enabled
dynamically.  Basically we'd need a way for the simpletrace backend
to output its data to a fifo, instead of creating an file on disk,
then you could dynanically consume it.


Regards,
Daniel
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