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

[Don Slutz <dslutz@verizon.com>]

On 07/11/13 08:07, Paul Durrant wrote:
>> -----Original Message-----
>> From: Ian Campbell [mailto:ian.campbell@citrix.com]
>> Sent: 11 July 2013 12:44
>> To: Paul Durrant
>> Cc: xen-devel@lists.xen.org
>> 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);
I handle this by adjusting device_model_args_hvm like:

device_model_args_hvm = [ "-trace", "events=/etc/xen/qemu-trace-events" ]

in the config.
>> 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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