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Re: [Qemu-devel] [PATCH v2 0/5] backdoor: lightweight guest-to-QEMU back

From: Blue Swirl
Subject: Re: [Qemu-devel] [PATCH v2 0/5] backdoor: lightweight guest-to-QEMU backdoor channel
Date: Thu, 8 Dec 2011 20:45:59 +0000

On Wed, Dec 7, 2011 at 18:54, Anthony Liguori <address@hidden> wrote:
> On 12/07/2011 12:51 PM, Peter Maydell wrote:
>> 2011/12/7 Lluís Vilanova<address@hidden>:
>>> Anthony Liguori writes:
>>> [...]
>>>> Why should this analyzer live outside of QEMU in the first place?  I
>>>> fail to see
>>>> the rationale for that other than not wanting to do the work of making
>>>> it
>>>> suitable for upstream consumption.
>>> For the same reason that SystemTap lets you add user-provided code into
>>> the
>>> trace hooks, you never know what the user actually wants. The difference
>>> is that
>>> when dealing with traces that require a high bandwidth, different people
>>> may be
>>> interested on analyzing different events and under different conditions,
>>> and
>>> having a JIT in QEMU comes in handy to optimize the traces. This includes
>>> the
>>> generation of extra tracing code at translation time (e.g., I generate
>>> checks in
>>> TCG to establish when I want to trace a specific event, and someone else
>>> may
>>> just want to increment a counter using TCG code).
>>> Guest trace instrumentation turns QEMU into a highly-performant tool for
>>> dynamic
>>> binary instrumentation, with all the benefits that stem from QEMU
>>> (support for a
>>> myriad of target architectures, as well as support for both full-system
>>> and
>>> user-level applications).
>> I think this *is* useful. However I also think that it *is* effectively
>> defining an API for people writing this hook code, and as such we ought
>> to do it properly if we're going to do it. (ie nail down what we are
>> providing for hook authors and don't let them grub around in the internals
>> otherwise).
> I strongly suspect that you could define interfaces in QEMU such that you
> could do most of what you want without needing to link any code against QEMU
> itself.

I don't see linking as the problem, the instrumenting user who
modifies QEMU needs to follow licensing if the result is ever

But the API issue can be a problem. Reusing and extending tracepoints
should help API stability from an instrumenting user point of view and
a rich set of various static and dynamic tracepoint mechanisms should
only be helpful for development of QEMU. But also I agree that the
internals of QEMU shouldn't become an API for any user code which
happens to access them.

> This is probably a case where LUA integration might make a lot of sense.
> Regards,
> Anthony Liguori
>> -- PMM

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