Re: [Qemu-devel] [PATCH 00/13] instrument: Add basic event instrumentation

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Tue, Aug 01, 2017 at 02:54:29PM +0100, Peter Maydell wrote:
> On 1 August 2017 at 14:48, Stefan Hajnoczi <stefanha@gmail.com> wrote:
> > Thanks for sharing the requirements.  A stable API is necessary for
> > providing these features.
> >
> > We're essentially talking about libqemu.  That means QEMU in library
> > form with an API for JIT engine, reverse engineering, instrumentation,
> > etc tasks.
> 
> > Maintaining libqemu will take ongoing effort and no one has committed.
> > The last discussion about libqemu was here:
> > https://lists.gnu.org/archive/html/qemu-devel/2016-11/msg04847.html
> 
> That thread seems to be focused on trying to extract TCG
> from the rest of QEMU, which definitely isn't a requirement
> for instrumentation, and I would suggest is something
> of a distraction from it.

You are right, it's a different use case.  I just wanted to reference
previous discussion about libqemu.  There have been more in the past for
different use cases.

> I want to be able to say "just
> instrument this setup that QEMU already provides as a
> board model", not have to write a driver that duplicates
> all the work vl.c and our board models do for us today,

Calling qemu_init(argc, argv) and qemu_run() isn't too onerous.  For the
price of that you get an environment where we can offer stable APIs.

> and I don't need the TCG engine to be a library to do that...

You do need TCG APIs if you want TCG-level instrumentation, tuning
options, callbacks, etc.

> I agree that we want to provide something that is at least
> closer to a stable API than "just expose trace events",
> though.

libqemu has at least three parts:

1. VM API (i.e. qemu_init(argc, argv), qemu_run(), qemu_vcpu_get_reg32())
2. TCG engine
3. Device models

Like I said in my email, start with what matters for the instrumentation
use case (VM API at a minimum to control guest execution).  Other people
can flesh out the other parts later, as needed.

Other attempts to provide a stable API will be essentially the same
thing as libqemu.

Stefan

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