From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:59815) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1db6ua-0003bg-8x for qemu-devel@nongnu.org; Fri, 28 Jul 2017 11:10:45 -0400 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1db6uX-0004or-11 for qemu-devel@nongnu.org; Fri, 28 Jul 2017 11:10:44 -0400 Received: from roura.ac.upc.es ([147.83.33.10]:42222) by eggs.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1db6uW-0004m5-L9 for qemu-devel@nongnu.org; Fri, 28 Jul 2017 11:10:40 -0400 From: =?utf-8?Q?Llu=C3=ADs_Vilanova?= 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> <20170727154535.GY2555@redhat.com> <20170728133430.GS12364@stefanha-x1.localdomain> Date: Fri, 28 Jul 2017 18:10:25 +0300 In-Reply-To: <20170728133430.GS12364@stefanha-x1.localdomain> (Stefan Hajnoczi's message of "Fri, 28 Jul 2017 14:34:30 +0100") Message-ID: <87r2x0ppge.fsf@frigg.lan> MIME-Version: 1.0 Content-Type: text/plain 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: Stefan Hajnoczi Cc: "Daniel P. Berrange" , QEMU Developers , Peter Maydell , "Emilio G. Cota" , Stefan Hajnoczi Stefan Hajnoczi writes: > On Thu, Jul 27, 2017 at 04:45:35PM +0100, Daniel P. Berrange wrote: >> 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. > One exception is gdb's static userspace probes support. If you can run > gdb on QEMU then you can trace the same events as SystemTap. I have > never tried this GDB feature: > https://sourceware.org/gdb/onlinedocs/gdb/Static-Probe-Points.html > It should work out of the box if your distro builds QEMU with the > 'dtrace' backend enabled. I tried it once long time ago and didn't work for me. Maybe I just missed something. Lluis