[libvirt] RFC: Libvirt extensions for QEMU tracing

[Daniel P. Berrange]

On Tue, Oct 26, 2010 at 12:15:30PM +0530, Prerna Saxena wrote:
> Hi,
> Tracing infrastructure is now a part of upstream QEMU, which allows 
> options to choose different trace backends to handle trace-events of 
> interest. The choice of 'simple' trace backend allows users to 
> dynamically enable/disable trace events for a running qemu instance as 
> well as to set options for logging traces to a desired file via the qemu 
> monitor.
> While the QMP interfaces are still under discussion 
> (http://www.mail-archive.com/qemu-devel@nongnu.org/msg44535.html), I 
> propose the following extensions for virsh to make use of human-monitor 
> tracing commands:
> 
> 1. virsh trace-events DOMAIN-ID [set TRACE-EVENT ON ]
> ----------------------------------------------------
> 	- Implements QEMU monitor command 'trace event' to change state of a 
> particular trace-event.
> 	- Eg, virsh trace-events DOMAIN-ID set ABC on
> 		: changes state of trace-event 'ABC' to enabled.
> 	- When specified without arguments, it implements QEMU monitor 
> 	command to show all currently available trace events and their state for a 
> specific instance.
> 	- Eg, virsh trace-events DOMAIN-ID
> 		: lists all trace-events with their state for that instance.
> 
> 2. virsh trace-file DOMAIN_ID [set FILENAME | --enable | --disable]
> -------------------------------------------------------------------
> 	- Implements the qemu monitor command : trace-file
> 	- Without any arguments, it lists the currently active trace output 
> file with its state.
> 	- The 'set' subcommand changes the output file to FILENAME.
> 	- The --enable and --disable switches respectively enable and 
> 	disable writing of trace data to output file.
> 
> The catch is, these are only available for 'simple' trace backend for 
> qemu, so one would need to be careful of handling failures when these 
> commands are passed to qemu instances compiled with a different trace 
> backend.

As such I'm not really convinced this is going to be useful. I don't
really see distros choosing to build the simple trace backend, when
the other backends (LTT-NG, or soon DTrace) are so much more flexible
and functional. Certainly in both Fedora and RHEL we'd just go for
a DTrace/SystemTAP tracing backend because that gives you end-to-end
tracing across the entire stack (virt-manager, libvirt, qemu/kvm
and kernel), as opposed to an isolated QEMU specific tool.

Regards,
Daniel
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