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

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

On Thu, Jul 27, 2017 at 11:54:29AM +0100, Peter Maydell wrote:
> On 27 July 2017 at 11:43, Daniel P. Berrange <berrange@redhat.com> wrote:
> > Maybe I'm missing something, but aren't all these things
> > already possible via either the statically defined tracepoints
> > QEMU exposes, or by placing dynamic tracepoints on arbitrary
> > code locations using dtrace/systemtap/lttng-ust.
> 
> Last time I looked we didn't have tracepoints on most of
> the events you'd be interested in.
> 
> 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.

You can't typically run this as root, 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.

If the goal is to be easy to instrument without havig to rebuild
QEMU, then I think using one of the existing trace backends is
the best viable option, as those are already enabled by distros.
I find it very unlikely that Fedora/RHEL would ever enable a
trace backend that lets you load arbitrary code into the QEMU
process, so you'd be back to having to rebuild QEMU again even
with that approach.


Regards,
Daniel
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