On 2011-08-31 11:08, Stefan Hajnoczi wrote: > 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.
We already have isa-debugcon or plain serial/virtio consoles. Should be easy to derive e.g. some mmio-debugcon and to establish a chardev backend that writes out trace events. I also think that fw_cfg is for, well, configuration. Jan -- Siemens AG, Corporate Technology, CT T DE IT 1 Corporate Competence Center Embedded Linux
