[Qemu-devel] Tracing: outstanding tasks

[Stefan Hajnoczi]

Here are the outstanding tasks for QEMU tracing, which Prerna and I have
been working on.  Tracing aids debugging, profiling, and observing
execution via lightweight logging at key points in the code path.

The current prototype is available from the 'tracing' branch at:

http://repo.or.cz/w/qemu/stefanha.git/shortlog/refs/heads/tracing

This email is both to help Prerna and me focus our efforts, as well as a
roadmap for the QEMU community to discuss and comment on.

Here are the outstanding tasks for a tracing patchset that can be
proposed for merge:

1. Integration with QMP

Owner: Prerna

Currently the trace commands are available from the monitor but a QMP interface
is needed.

2. More tracepoints need to be added for instrumenting other qemu components
such as virtio drivers, etc.

Owner: ?

QEMU must come with a useful set of trace events that allows people to get up
and running quickly.  Focus areas include:

 * guest device emulation
 * host devices
 * lifecycle and runloop
 * memory management
 * live migration

3. Documentation

Owner: Stefan

User documentation that explains how to collect traces and add new trace
events.  I have committed documentation to the tracing branch here:

http://repo.or.cz/w/qemu/stefanha.git/blob_plain/91fde34dc6bfe01af6d5e9265f6a81535d6add15:/docs/tracing.txt

4. Fix i386-linux-user build

Owner: Prerna

  LINK  i386-linux-user/qemu-i386
../simpletrace.o: In function `do_info_all_trace_events':
/home/stefanha/qemu/simpletrace.c:88: undefined reference to `monitor_printf'
../simpletrace.o: In function `do_info_trace':
/home/stefanha/qemu/simpletrace.c:77: undefined reference to `monitor_printf'

5. Out-of-line trace file write-out

Owner: Stefan

Trace buffers are written out to file synchronously.  The vcpu thread should
not be blocked so an async write-out mechanism is needed.

6. Trace file command

Owner: ?

Traces are written out to hardcoded /tmp/trace.log.  This must be configurable.
Tracing at startup time should still be possible so configuration needs to
happen early.

7. Binary trace format finalization

Owner: Stefan

We should leave room for extension.  I suggest partitioning the Event ID
namespace into normal events and special events.  The __trace_begin special
event is defined to contain the file format version and/or trace record size in
bytes as the first trace record in the file.

This way, post-processing tools can check the format of the binary trace file.

8. QMP/monitor command review

Owner: Prerna, Stefan

Are we happy to support the trace commands in the longer term (i.e. did we get
the commands and syntax right)?  Is input being validated properly?

Stefan