On Wed, Aug 31, 2011 at 6:38 AM, Stefan Hajnoczi <stefa...@gmail.com> wrote: > 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? >
I will try to post it soon. It will probably be untested and will not work, but you will have the approach to look at :-) Thanks Dhaval
