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

From: Stefan Hajnoczi <stefanha@redhat.com>
To: Peter Maydell <peter.maydell@linaro.org>
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 15:49:43 +0100	[thread overview]
Message-ID: <20170802144943.GF10101@stefanha-x1.localdomain> (raw)
In-Reply-To: <CAFEAcA82EBmOF3k=fXN4tAAL2=J6s8avUoQGXKrBwchh1rmMCQ@mail.gmail.com>

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On Wed, Aug 02, 2017 at 12:10:14PM +0100, Peter Maydell wrote:
> 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.

Please post an example of the API you'd like.

Stefan

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