On Thu, 18 Aug 2016 14:53:27 +0100 Stefan Hajnoczi <stefa...@redhat.com> wrote:
> On Thu, Aug 18, 2016 at 12:22:18PM +0200, Lluís Vilanova wrote: > > Stefan Hajnoczi writes: > > > > > On Fri, Aug 05, 2016 at 06:59:23PM +0200, Lluís Vilanova wrote: > > >> The hypertrace channel allows guest code to emit events in QEMU (the > > >> host) using > > >> its tracing infrastructure (see "docs/trace.txt"). This works in both > > >> 'system' > > >> and 'user' modes. That is, hypertrace is to tracing, what hypercalls are > > >> to > > >> system calls. > > >> > > >> You can use this to emit an event on both guest and QEMU (host) traces > > >> to easily > > >> synchronize or correlate them. You could also modify you guest's tracing > > >> system > > >> to emit all events through the hypertrace channel, providing a unified > > >> and fully > > >> synchronized trace log. Another use case is timing the performance of > > >> guest code > > >> when optimizing TCG (QEMU traces have a timestamp). > > >> > > >> See first commit for a full description. > > >> > > >> Signed-off-by: Lluís Vilanova <vilan...@ac.upc.edu> > > >> --- > > > > > CCing Steven Rostedt, Masami Hiramatsu, Luiz Capitulino, and LTTng folks > > > who have all looked into host/guest tracing solutions. > > [...] > > > > Oh, I wasn't aware of that. I'm certainly interested in collaborating. > > They are working on or have worked on different approaches to host/guest > tracing. Unfortunately there isn't an out-of-the-box solution as far as > I know. The ftrace solution is documented here: https://lists.nongnu.org/archive/html/qemu-devel/2016-03/msg00887.html This traces the guest and host kernels. It supports merging the guest and host traces. It's extremely low latency and has helped us to find several spikes for real-time KVM (we're talking a few to a dozen microseconds at most). Now, our stack actually is: - Guest app - Guest kernel - Host kernel - QEMU QEMU already has its own tracing (which I don't know how it works). If I had to trace the guest app, I'd certainly start off by using LTTng. Although, we'd have to write a tool to merge and orchestrate (wooo, cloud buzzword!) all those traces (if that's what one wants). > It would be nice if there was a documented host/guest tracing approach > that didn't involve much manual setup and handled most use cases. I'd volunteer to do it, although it will take me weeks to do it.
