Re: [PATCH v2 3/3] trace: recommend "log" backend for getting started with tracing

[Stefan Hajnoczi]

On Thu, Dec 31, 2020 at 09:36:50PM +0100, BALATON Zoltan wrote:
> On Thu, 31 Dec 2020, Peter Maydell wrote:
> > On Wed, 16 Dec 2020 at 16:09, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > > 
> > > The "simple" backend is actually more complicated to use than the "log"
> > > backend. Update the quickstart documentation to feature the "log"
> > > backend instead of the "simple" backend.
> > > 
> > > Suggested-by: Peter Maydell <peter.maydell@linaro.org>
> > > Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> > > ---
> > >  docs/devel/tracing.rst | 35 ++++++++++++++++++-----------------
> > >  1 file changed, 18 insertions(+), 17 deletions(-)
> > > 
> > > diff --git a/docs/devel/tracing.rst b/docs/devel/tracing.rst
> > > index 76cc1b24fa..e60058bf55 100644
> > > --- a/docs/devel/tracing.rst
> > > +++ b/docs/devel/tracing.rst
> > > @@ -11,22 +11,22 @@ for debugging, profiling, and observing execution.
> > >  Quickstart
> > >  ==========
> > > 
> > > -1. Build with the 'simple' trace backend::
> > > +Enable tracing of ``memory_region_ops_read`` and 
> > > ``memory_region_ops_write``
> > > +events::
> > > 
> > > -    ./configure --enable-trace-backends=simple
> > > -    make
> > > +    $ qemu --trace "memory_region_ops_*" ...
> > > +    ...
> > > +    719585@1608130130.441188:memory_region_ops_read cpu 0 mr 
> > > 0x562fdfbb3820 addr 0x3cc value 0x67 size 1
> > > +    719585@1608130130.441190:memory_region_ops_write cpu 0 mr 
> > > 0x562fdfbd2f00 addr 0x3d4 value 0x70e size 2
> > > 
> > > -2. Create a file with the events you want to trace::
> > > +This output comes from the "log" trace backend that is enabled by 
> > > default when
> > > +``./configure --enable-trace-backends=BACKENDS`` was not explicitly 
> > > specified.
> > > 
> > > -    echo memory_region_ops_read >/tmp/events
> > > +More than one trace event pattern can be specified by providing a file
> > > +instead::
> > 
> > Does --trace really not let you specify more than one pattern
> > without resorting to putting them into a file? That sounds like a
> > deficiency compared to -d (which allows -d trace:PATTERN,trace:PATTERN...)
> > that we could look at fixing...
> 
> Ir's possible to give more patterns in multiple options, I'm using that,
> such as:
> 
> -trace enable="pci*" -trace enable="ide*"
> 
> For a lot of patterns using a file may be clearer though.
> 
> This reminds me to the plainlog backend I've submitted. What happened to
> that? See:
> 
> https://lists.nongnu.org/archive/html/qemu-devel/2020-06/msg07296.html
> 
> I'd like a solution that can be set at compile time and does not need
> another command line option to turn off time stamps. (Timestamps are
> annoyong when comparing logs that's often necessary to check changes.) I'm
> still using my plainlog patch but I have to disable that when bisecting.

Thanks for reminding me. I don't remember what happened to that, sorry!

I will revisit the plainlog series and let you know.

Stefan

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