Re: [Qemu-devel] [PATCH, RFC] trace: implement guest tracepoint passthrough

[Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>]

On Wed, Aug 31, 2011 at 11:38:50AM +0300, Avi Kivity wrote:
> On 08/26/2011 10:06 PM, Blue Swirl wrote:
> >Let guests inject tracepoint data via fw_cfg device.
> >
> >
> 
> At least on x86, fw_cfg is pretty slow, involving multiple exits.
> IMO, for kvm, even one exit per tracepoint is too high.  We need to
> use a shared memory transport with a way to order guest/host events
> later on (by using a clock).

It depends how you want to use this.  If you need it for guest firmware
debugging or bringing up a new target, then this approach is fine.

But this is not a mechanism that is suitable for performance analysis or
production tracing (the fact that the QEMU and guest software need to be
built together in order to sync on event IDs is the killer).

Dhaval is looking at Linux guest tracing which is suitable for
performance work.  This does not necessarily involve modifying QEMU.
Currently he uses a hypercall but a virtio device would be possible too.
The key thing is that it integrates with the host kernel tracing
infrastructure so you get a unified trace instead of terminating in QEMU
userspace.

So I see Blue's feature as a quick starting point for people who need to
debug and hack guests.  It should be simple and easy to get going for
QEMU developers, but is not suitable for other use.

Stefan