Re: [Xen-devel] [PATCH] Plumb through xen-platform device logging

[Don Slutz <dslutz@xxxxxxxxxxx>]

On 07/11/13 08:07, Paul Durrant wrote:
> > -----Original Message-----
> > From: Ian Campbell [mailto:ian.campbell@xxxxxxxxxx]
> > Sent: 11 July 2013 12:44
> > To: Paul Durrant
> > Cc: xen-devel@xxxxxxxxxxxxx
> > Subject: Re: [Xen-devel] [PATCH] Plumb through xen-platform device logging
> >
> > On Thu, 2013-07-11 at 08:23 +0000, Paul Durrant wrote:
> > > > diff --git a/tools/Makefile b/tools/Makefile
> > > > index e44a3e9..2d791a4 100644
> > > > --- a/tools/Makefile
> > > > +++ b/tools/Makefile
> > > > @@ -202,7 +202,8 @@ subdir-all-qemu-xen-dir: qemu-xen-dir-find
> > > >                 --disable-kvm \
> > > >                 --disable-docs \
> > > >                 --python=$(PYTHON) \
> > > > -               $(IOEMU_CONFIGURE_CROSS); \
> > > > +               $(IOEMU_CONFIGURE_CROSS) \
> > > > +               --enable-trace-backend=stderr; \
I have done this in a more flexable way:

--- a/tools/Makefile
+++ b/tools/Makefile

@@ -187,8 +185,7 @@ subdir-all-qemu-xen-dir: qemu-xen-dir-find
                source=.; \
        fi; \
        cd qemu-xen-dir; \
-       $$source/configure --enable-xen --target-list=i386-softmmu \
-               --prefix=$(PREFIX) \
+       $$source/configure --enable-xen $(QEMU_UPSTREAM_EXTRA_CONFIG) 
--target-list=i386-softmmu \
                --source-path=$$source \
                --extra-cflags="-I$(XEN_ROOT)/tools/include \
                -I$(XEN_ROOT)/tools/libxc \

I.E by adding QEMU_UPSTREAM_EXTRA_CONFIG which I add to .config when I 
want to turn on tracing.
> > Is this the only way to do this?
> >
> > It is likely that many distros will use their standard qemu packages
> > which may or may not have this option enabled.
> That's true.
>   
> > I'm not sure if this is a static always on option or if this is simply
> > making a trace backend available for use.
> It's a static selection as far as I can tell. The tracing backend is not 
> selectable at runtime :-(
It is static and adds a lot of code.  So you want a way to not configure 
it also.  However what trace output happens is changeable while running.
> > Looking at http://wiki.qemu.org/Features/Tracing is the tracing
> > interface really the right way to be logging this particular class of
> > information? I'd have thought a simple logfile support in the platform
> > device would be a much more natural fit.
> That makes sense to me, but whoever coded up the platform device obviously 
> believed tracing to be the correct way to log. I don't know the history of that 
> decision.
> It doesn't change the fact though that, currently, xen builds of QEMU don't 
> configure any form of tracing backend at all which doesn't seem particularly 
> helpful, and I did introduce platform logging as an example of an event to log 
> so I think the patch is useful as far as it goes, but maybe another patch to 
> the platform device in QEMU would also be considered a good idea.
>
>    Paul
>
> > > >         $(MAKE) all
> > > >
> > > > diff --git a/tools/libxl/libxl_dm.c b/tools/libxl/libxl_dm.c
> > > > index 2924298..35f71cc 100644
> > > > --- a/tools/libxl/libxl_dm.c
> > > > +++ b/tools/libxl/libxl_dm.c
> > > > @@ -370,6 +370,13 @@ static char **
> > > > libxl__build_device_model_args_new(libxl__gc *gc,
> > > >                         "-xen-domid",
> > > >                         libxl__sprintf(gc, "%d", guest_domid), NULL);
> > > >
> > > > +    flexarray_vappend(dm_args,
> > > > +                      "-trace",
> > > > +                      libxl__sprintf(gc,
> > > > +                                     "events=%s/qemu-trace-events",
> > > > +                                     libxl__xen_config_dir_path()),
> > > > +                      NULL);
> > Doesn't this end up logging to /etc/xen? Not what we want I think.
> >
> > or maybe this is just the config file, which, apart from my comments
> > about the suitability of the interface above, makes me wonder where the
> > logs do go? Ideally they would go to /var/log/xen/qemu-blah-name.log not
> > to xl stdout.
Using --enable-trace-backend=stderr, the output ends up in 
/var/log/xen/qemu-dm-P-1-0.log
I.E. where all stderr from qemu does.
   -Don Slutz
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