From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:53512) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1dakyy-0003K3-72 for qemu-devel@nongnu.org; Thu, 27 Jul 2017 11:45:49 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1dakyu-0002UQ-PV for qemu-devel@nongnu.org; Thu, 27 Jul 2017 11:45:48 -0400 Received: from mx1.redhat.com ([209.132.183.28]:31783) by eggs.gnu.org with esmtps (TLS1.0:DHE_RSA_AES_256_CBC_SHA1:32) (Exim 4.71) (envelope-from ) id 1dakyu-0002Tz-Fd for qemu-devel@nongnu.org; Thu, 27 Jul 2017 11:45:44 -0400 Date: Thu, 27 Jul 2017 16:45:35 +0100 From: "Daniel P. Berrange" Message-ID: <20170727154535.GY2555@redhat.com> Reply-To: "Daniel P. Berrange" References: <150091574424.30739.4131793221953168474.stgit@frigg.lan> <20170725131931.GC23343@stefanha-x1.localdomain> <20170726112633.GE18489@stefanha-x1.localdomain> <20170727104302.GI2555@redhat.com> <20170727152137.GW2555@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: Subject: Re: [Qemu-devel] [PATCH 00/13] instrument: Add basic event instrumentation List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Peter Maydell Cc: Stefan Hajnoczi , Stefan Hajnoczi , "Emilio G. Cota" , =?utf-8?B?TGx1w61z?= Vilanova , QEMU Developers On Thu, Jul 27, 2017 at 04:33:01PM +0100, Peter Maydell wrote: > On 27 July 2017 at 16:21, Daniel P. Berrange 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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|