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

[Peter Maydell]

On 2 August 2017 at 12:04, Stefan Hajnoczi <address@hidden> wrote:
> On Tue, Aug 01, 2017 at 02:54:29PM +0100, Peter Maydell wrote:
>> 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 need an API; that doesn't necessarily look like the kind
of API you want to be able to embed the TCG engine into
other things, I think.

>> 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.

I don't think this is the case -- you could have a stable
instrumentation API without it looking anything like
libqemu. In particular I don't think you need to have
something that sits at the top level and says 'run'.

In particular I think that pulling TCG out of QEMU
is an enormous and painful undertaking that you just
don't need to do at all to allow this kind of
instrumentation API.

thanks
-- PMM