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

From: Peter Maydell <peter.maydell@linaro.org>
To: Stefan Hajnoczi <stefanha@redhat.com>
Cc: "Stefan Hajnoczi" <stefanha@gmail.com>,
	"Lluís Vilanova" <vilanova@ac.upc.edu>,
	"Alessandro Di Federico" <ale+qemu@clearmind.me>,
	"Daniel P. Berrange" <berrange@redhat.com>,
	"Emilio G. Cota" <cota@braap.org>,
	"QEMU Developers" <qemu-devel@nongnu.org>
Subject: Re: [Qemu-devel] [PATCH 00/13] instrument: Add basic event instrumentation
Date: Wed, 2 Aug 2017 12:10:14 +0100	[thread overview]
Message-ID: <CAFEAcA82EBmOF3k=fXN4tAAL2=J6s8avUoQGXKrBwchh1rmMCQ@mail.gmail.com> (raw)
In-Reply-To: <20170802110400.GI5531@stefanha-x1.localdomain>

On 2 August 2017 at 12:04, Stefan Hajnoczi <stefanha@redhat.com> 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