Re: [Qemu-devel] [PATCH 0/9] simplify usage of tracepoints, and connect them to logging

[Peter Maydell <peter.maydell@linaro.org>]

On 25 October 2015 at 13:32, Paolo Bonzini <pbonzini@redhat.com> wrote:
> This series does three things:
>
> 1) add a "-trace [enable=]foo" option to enable one or more trace
> events, and a "-trace help" option to show the list of tracepoints
> (patches 4-5)
>
> 2) change the stderr tracing backend so that it prints to the
> -D log file, and enable it by default.  "-trace file=..." is
> now a synonym of -D if the log backend is enabled (patches 7-8)
>
> 3) add a "-d trace:foo" option that is a synonym for "-trace foo";
> this makes the new functionality more discoverable to people used
> to "-d", makes it available for user-mode emulation (which does
> not have -trace), and is somewhat nice if you want to enable both
> tracepoints and some other "-d" flag (patch 9).  When globbing
> it is also less susceptible to unwanted shell expansion.
>
> For example, you can trace block device I/O and save the result
> to a file just by adding "-trace bdrv_aio_*,file=trace.txt", or
> correlate it to guest PCs with "-d exec,nochain,trace:bdrv_aio_*".
>
> Opinions?  I would like to have this in 2.5 if there is agreement.

Have you done any performance testing to check that we don't have
previously-nopped-out tracepoints in hot paths that now result in
real code being generated?

thanks
-- PMM