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

[Blue Swirl <blauwirbel@gmail.com>]

On Mon, Aug 29, 2011 at 12:51 PM, Lluís <xscript@gmx.net> wrote:
> 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.

Well, fw_cfg is intended for very low level BIOS code. Virtual PCI
devices would be better for OS level.

> 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.

This should be possible.