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

[Lluís <xscript@gmx.net>]

Stefan Hajnoczi writes:

> The ability to trace from the guest can be handy, so I think we should
> have this feature.  Please add documentation on how to hook it up
> (e.g. how people would use this for other firmware/guest code and/or
> other architectures).

> Guest and QEMU need to agree on event IDs.  The guest code needs to be
> built with QEMU and they may not function with other QEMU builds or
> guest builds.  This is fine for development but not feasible when QEMU
> and the guest code are built or provided separately.

> I suggest we merge this as a development feature that can be used when
> bringing up new architectures, debugging guest code, or for some types
> of performance work.  This feature falls under the Do-It-Yourself
> area, where things could break relatively easy but developers who wish
> to use it should be able to get it working in their area.

This sounds is indeed interesting.

I suppose I could even use this as a backdoor mechanism for the guest to
communicate with QEMU. The only problem I see is that fw_cfg is one-way
(i.e., QEMU cannot return any data back to the guest), as opposed to a
backdoor mechanism based on a virtual device or "magic" instructions.


Lluis

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