Re: [Xen-devel] [PATCH] tools: probe for existence of qemu-xen trace backends.

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Wed, 10 Feb 2016, Ian Campbell wrote:
> On Wed, 2016-02-10 at 14:51 +0000, Stefano Stabellini wrote:
> > On Wed, 10 Feb 2016, Ian Campbell wrote:
> > > On Wed, 2016-02-10 at 14:27 +0000, Ian Campbell wrote:
> > > > On Wed, 2016-02-10 at 14:18 +0000, Anthony PERARD wrote:
> > > > > On Wed, Feb 10, 2016 at 02:14:13PM +0000, Stefano Stabellini wrote:
> > > > > > On Wed, 10 Feb 2016, Ian Campbell wrote:
> > > > > > > diff --git a/tools/Makefile b/tools/Makefile
> > > > > > > index 5688a7c..76a2235 100644
> > > > > > > --- a/tools/Makefile
> > > > > > > +++ b/tools/Makefile
> > > > > > > @@ -228,7 +228,7 @@ qemu-xen-dir-force-update: qemu-xen-dir-
> > > > > > > find
> > > > > > > Â fi
> > > > > > > Â
> > > > > > > Âifeq ($(debug),y)
> > > > > > > -QEMU_XEN_ENABLE_DEBUG := --enable-debug --enable-trace-
> > > > > > > backend=stderr
> > > > > > > +QEMU_XEN_ENABLE_DEBUG := --enable-debug
> > > > > > > Âelse
> > > > > > > ÂQEMU_XEN_ENABLE_DEBUG :=
> > > > > > > Âendif
> > > > > > > @@ -240,8 +240,16 @@ subdir-all-qemu-xen-dir: qemu-xen-dir-find
> > > > > > > Â         source=.; \
> > > > > > > Â fi; \
> > > > > > > Â cd qemu-xen-dir; \
> > > > > > > + if $$source/scripts/tracetool.py --check-backends --
> > > > > > > backends
> > > > > > > log ; then \
> > > > > >
> > > > > > --check-backends only works on qemu-xen >= 4.6, on the other hand
> > > > > > we
> > > > > > know that qemu-xen < 4.6 supports stderr.
> > > > >
> > > > > But, if you use '--check-backend --backend' (no 's') instead, the
> > > > > check
> > > > > would works with qemu-xen >= 4.3
> > > >
> > > > That would be preferable to the below, IMHO. Any contrary opinions?
> > >
> > > Actually, given that the upstream default is now "log", how about:
> > >
> > > Â Â Â Â if $$source/scripts/tracetool.py --check-backend --backend
> > > stderr ; then \
> > >           enable_trace_backend='--enable-trace-backend=stderr'; \
> > >   else \          enable_trace_backend='' ; \
> > >
> > > Â Â Â Â fi ; \
> > >
> > > IOW just accept the upstream default (be it log or whatever comes next)
> > > from this version onwards?
> >
> > The risk associated with that is that in the past QEMU recommended
> > 'simple', which has a binary output to a separate file.
>
> I can't see any evidence of that in the git log over configure (which has
> trace_backend="<thing>" in it. The configure file gained this option in
> "trace: Add trace-events file for declaring trace events" with "nop" as the
> default (circa QEMU 0.13.50) which it was until the recent switch to "log".

From docs/tracing.txt:

"The "simple" backend supports common use cases and comes as part of the QEMU
source tree.  It may not be as powerful as platform-specific or third-party
trace backends but it is portable.  This is the recommended trace backend
unless you have specific needs for more advanced backends."


> In any case what harm does have some other upstream default enable do? AIUI
> you need to also enable specific tracing on the QEMU command line for it to
> be useful, surely no version of QEMU was dumping some binary trace file
> into $CWD by default?

This is true

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