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Date: Thu, 7 Sep 2017 10:21:52 -0400
From: "Emilio G. Cota" <cota@braap.org>
Message-ID: <20170907142152.GA19705@flamenco>
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Subject: Re: [Qemu-devel] [PATCH v4 00/20] instrument: Add basic event
 instrumentation
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To: Markus Armbruster <armbru@redhat.com>
Cc: =?iso-8859-1?Q?Llu=EDs?= Vilanova <vilanova@ac.upc.edu>, qemu-devel@nongnu.org, Stefan Hajnoczi <stefanha@redhat.com>

On Thu, Sep 07, 2017 at 12:58:05 +0200, Markus Armbruster wrote:
> Lluís Vilanova <vilanova@ac.upc.edu> writes:
> 
> > This series adds an API to add instrumentation events.
> >
> > It also provides additional APIs for:
> > * Controlling tracing events
> > * Peek/poke guest memory
> >
> > There's still missing APIs for (can be added in later series?):
> > * Provide something like tracing's per-vCPU trace states (i.e., so that each
> >   vCPU can have different instrumentation code). It's still not clear to me if
> >   we should extend the per-vCPU bitmap with instrumentation events, or otherwise
> >   somehow reuse the bits in tracing events (since they're currently limited).
> > * Peek/poke guest registers
> >
> > The instrumentation code is dynamically loaded as a library into QEMU either
> > when it starts or later using its remote control interfaces.
> >
> > Signed-off-by: Lluís Vilanova <vilanova@ac.upc.edu>
> 
> Taking a step back.
> 
> This looks like a way to dynamically load arbitrary code.  What
> interfaces can this code use?  Your cover letter should answer this.
> 
> As long as the answer is "everything the dynamic linker is willing to
> resolve", this series heading nowhere.  We can talk about an interface
> for plugins, but "anything goes" is not on the menu.

A simple solution to this is to only export the API symbols by passing
--dynamic-file to the linker -- see patch 2 of the following series for an
example (ELF-only, although I'm pretty sure this can be achieved on Windows
as well):
  https://lists.gnu.org/archive/html/qemu-devel/2017-09/msg01446.html

Thanks,

		Emilio