On Wed, Aug 31, 2011 at 10:11 AM, Avi Kivity <a...@redhat.com> wrote: > On 08/31/2011 12:08 PM, Stefan Hajnoczi wrote: >> >> > >> > 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. > > IMO a hypercall is the way to go, virtio is not entirely suitable for > per-cpu work. > >> 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 should have one tracing mechanism that is useful everywhere, not > fragmented functionality.
You have a point. Dhaval: Any update on the approach you are working on? Do you have public code we can look at? Stefan