Re: [Qemu-devel] [PATCH] trace: teach lttng backend to use format strings

[Stefan Hajnoczi]

On Tue, Mar 25, 2014 at 02:49:42PM +0000, Alex Bennée wrote:
> 
> Stefan Hajnoczi <address@hidden> writes:
> 
> > On Mon, Mar 24, 2014 at 05:04:54PM +0000, address@hidden wrote:
> >> From: Alex Bennée <address@hidden>
> >> 
> >> This makes the UST backend pay attention to the format string arguments
> >> that are defined when defining payload data. With this you can now
> >> ensure integers are reported in hex mode if you want.
> >> 
> >> Signed-off-by: Alex Bennée <address@hidden>
> >> ---
> >>  scripts/tracetool/__init__.py    | 13 +++++++++++--
> >>  scripts/tracetool/backend/ust.py | 16 ++++++++++++----
> >>  2 files changed, 23 insertions(+), 6 deletions(-)
> >
> > How are strings handled, can LTTng copy them from the QEMU process'
> > memory space?
> 
> lttng provides ctf_string for passing strings to the payload. But I
> wouldn't expect to use them for the result of format string calculations
> as that would defeat the point of the low impact tracing.
> 
> There are a number of trace-events that pass strings for various things.
> I've not actually experimented with the output of any of them though.

Sounds like it should work but can you test it?  Try enabling the
bdrv_open_common() trace event when booting a guest.  You should see the
filename of the disk image file.

Stefan