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

From: "Daniel P. Berrange" <berrange@redhat.com>
To: Peter Maydell <peter.maydell@linaro.org>
Cc: "Stefan Hajnoczi" <stefanha@redhat.com>,
	"Stefan Hajnoczi" <stefanha@gmail.com>,
	"Emilio G. Cota" <cota@braap.org>,
	"Lluís Vilanova" <vilanova@ac.upc.edu>,
	"QEMU Developers" <qemu-devel@nongnu.org>
Subject: Re: [Qemu-devel] [PATCH 00/13] instrument: Add basic event instrumentation
Date: Fri, 28 Jul 2017 15:06:23 +0100	[thread overview]
Message-ID: <20170728140623.GQ31495@redhat.com> (raw)
In-Reply-To: <CAFEAcA9p38P2x3s4grvFPkGaH+JEmwqyAC9SsLWqqFL8k+Eiww@mail.gmail.com>

On Fri, Jul 28, 2017 at 02:41:19PM +0100, Peter Maydell wrote:
> On 28 July 2017 at 14:34, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> > Lluís/Peter: What are the requirements for instrumentation code
> > interacting with the running QEMU instance?  simpletrace is
> > asynchronous, meaning it does not wait for anyone handle the trace event
> > before continuing execution, and is therefore not suitable for
> > SystemTap-style scripts that can interact with the program while
> > handling a trace event.
> 
> I think you'd probably want synchronous -- it's pretty helpful
> to be able to say "register a trace event hook that doesn't
> fire very often, and use that to get to the region of
> execution that's of interest to you, then enable more hooks
> to get more detail at that point". (For instance, "wait til
> we've executed 5,000,000 instructions, then turn on the
> tracing of all instruction execution, register modification
> and memory accesses".)

Currently simpletrace probes have a fixed action when they are enabled,
namely to print state to the trace log file. Perhaps we can make the
action more flexible, if we create a more formal protocol for simpletrace
to let it talk over a UNIX socket. By default it could send probe data
asynchronously as now, but you could mark probes such that they require
a synchronous ACK, thus pausing execution until that ACK is received
from the instrumenting program.

For that to be useful, we would need to have allow probes to be turned
on/off via this trace socket, since the normal HMP/QMP monitor execution
would be blocked while this probe is running.

Regards,
Daniel
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