[lttng-dev] [Qemu-devel] [PATCH 0/6] hypertrace: Lightweight guest-to-QEMU trace channel

[Lluís Vilanova]

Luiz Capitulino writes:

> On Thu, 18 Aug 2016 14:53:27 +0100
> Stefan Hajnoczi <stefanha at 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 <vilanova at 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).
[...]

One of my targets was to simplify the merge by providing known reference points
between guest and host traces.


Cheers,
  Lluis