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

[Ian Campbell <ian.campbell@citrix.com>]

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
> > > > > >  	fi
> > > > > >  
> > > > > >  ifeq ($(debug),y)
> > > > > > -QEMU_XEN_ENABLE_DEBUG := --enable-debug --enable-trace-
> > > > > > backend=stderr
> > > > > > +QEMU_XEN_ENABLE_DEBUG := --enable-debug
> > > > > >  else
> > > > > >  QEMU_XEN_ENABLE_DEBUG :=
> > > > > >  endif
> > > > > > @@ -240,8 +240,16 @@ subdir-all-qemu-xen-dir: qemu-xen-dir-find
> > > > > >  		source=.; \
> > > > > >  	fi; \
> > > > > >  	cd qemu-xen-dir; \
> > > > > > +	if $$source/scripts/tracetool.py --check-backends --
> > > > > > backends 
> > > > > > log ; then \
> > > > > 
> > > > > --check-backends only works on qemu-xen >= 4.6, on the other hand
> > > > > we
> > > > > know that qemu-xen < 4.6 supports stderr.
> > > > 
> > > > But, if you use '--check-backend --backend' (no 's') instead, the
> > > > check
> > > > would works with qemu-xen >= 4.3
> > > 
> > > That would be preferable to the below, IMHO. Any contrary opinions?
> > 
> > Actually, given that the upstream default is now "log", how about:
> > 
> >         if $$source/scripts/tracetool.py --check-backend --backend
> > stderr ; then \
> > 		enable_trace_backend='--enable-trace-backend=stderr'; \
> > 	else \		enable_trace_backend='' ; \
> > 	
> >         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".

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?

Ian.

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