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

[Stefan Hajnoczi <stefanha@redhat.com>]

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On Fri, Jul 28, 2017 at 07:21:04PM +0300, Lluís Vilanova wrote:
> Stefan Hajnoczi writes:
> 
> > On Thu, Jul 27, 2017 at 11:40:17AM +0100, Peter Maydell wrote:
> >> On 27 July 2017 at 11:32, Stefan Hajnoczi <stefanha@redhat.com> wrote:
> >> > On Wed, Jul 26, 2017 at 03:44:39PM +0300, Lluís Vilanova wrote:
> >> >> And why exactly is this a threat? Because it can be used to "extend" QEMU
> >> >> without touching its sources? Is this a realistic threat? (it's a rather brittle
> >> >> way to do it, so I'm not sure it's practical)
> >> >
> >> > Unfortunately it is a problem.  I recently came across a product that
> >> > was using LD_PRELOAD= to "integrate" with QEMU.  People really abuse
> >> > these interfaces instead of integrating their features cleanly into
> >> > QEMU.
> >> 
> >> ...if people who want to do this kind of thing already can and
> >> do use LD_PRELOAD for it, I don't think we should worry too much
> >> about trying to make the instrumentation plugin API bulletproof
> >> against similar abuse.
> >> 
> >> > I see the use cases that Peter has been describing and am sure we can
> >> > come up with good solutions.  What I care about is that it doesn't allow
> >> > loading a .so that connects to arbitrary trace events.
> >> 
> >> That said, I agree that we don't really need an arbitrary-trace-event
> >> setup here, and we should probably design our API so that it isn't
> >> handing the trace plugin hooks pointers into QEMU's internals.
> >> We want an API that makes it easy for people to do things based on
> >> changes of the guest binary's state (registers, insns, etc etc)
> >> and which makes it hard for them to accidentally trip themselves up
> >> (eg by prodding around in QEMU internal data structures).
> >> This will have the secondary benefit that it's unlikely that future
> >> changes to QEMU will break plugin code.
> >> 
> >> >> As a side note, I find instrumentation to be most useful for guest code events,
> >> >> which mostly contain non-pointer values (except for the CPUState*).
> >> 
> >> For instance we definitely should not be passing a CPUState* to
> >> any plugin function.
> 
> > The gdbstub protocol has relevant features for accessing guest memory,
> > registers, etc.  Perhaps a set of QEMU-specific events can be added
> > (e.g. tb generated) so it's possible to instrument and control the
> > guest from an instrumentation program (written in any language).
> 
> > Perhaps there is a fundamental reason why this isn't possible due to the
> > protocol design, because using gdbstub halts all vcpus, etc.  I don't
> > know.
> 
> > Do you think this is an interesting direction?  It definitely seems like
> > a powerful approach though performance would be less than running native
> > code inside the QEMU process.
> 
> That's the same approach someone else dubbed as using a fifo with "synchronous"
> events, right? I have some measurements on this using a pipe, and overheads are
> 1000x to 2300x for each communication event (compared to a function call, and
> depending on whether each process/thread is pinned to the same or different
> CPU).

You are right.  I understand the need for native code without
interprocess communication now.

Stefan

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