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

From: Lluís Vilanova
Subject: Re: [Qemu-devel] [PATCH v2 0/5] backdoor: lightweight guest-to-QEMU backdoor channel
Date: Thu, 08 Dec 2011 23:16:52 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.0.92 (gnu/linux)

Blue Swirl writes:

> 2011/12/8 Lluís Vilanova <address@hidden>:
>> Stefan Hajnoczi writes:
>> [...]
>>>> * Support for tracing guest code through TCG.
>>> I'm not clear on whether a backdoor mechanism is needed or not.  A
>>> backdoor mechanism allows a modified guest to participate in tracing.
>>> Depending on the type of analysis you are doing it might be possible
>>> to achieve the same thing by emitting calls to custom instrumentation
>>> in TCG (or is this too low-level for what you're trying to trace in
>>> the guest?).  The advantage is also that you can do this to unmodified
>>> guests.
>> You're right with the first; it exists for two reasons:
>> * simplicity: it's easier to have, for example, a backdoor + linux 
>> tracepoints
>>  than obtaining the IP of an "interesting" function (think of the
>>  multiprogramming on a full system)
>> * performance: it's faster to let the guest tell you rather than (ab)using
>>  breakpoints or checking the IP of every instruction

> I still think that a breakpoint based system could be a useful method
> in addition to others, simply because it is entirely invisible to the
> guest and can then be used with unmodified and non-cooperating guests.
> Properly implemented, it should not have much overhead.

Yup. My plan was to implement this sometime in the future :)

On the performance side, there's only the degenerated case of a very hot code
page that repeatedly triggers false mmu faults due to the (never reached)

As for simplicity, I can use the backdoor to tell my analuzer to insert a
breakpoint in the given address, instead of having some complex system to get
symbol information from the outside (specially in softmmy mode).

>>> So I'm suggesting that we don't *need* explicit support for
>>> instrumenting trace events.  Instead add plain old functions where you
>>> need them.  You may decide to invoke trace events from your
>>> instrumentation function, but that's already covered today by
>>> docs/tracing.txt.
>>> Am I missing the point of instrumentation tracing events or do you
>>> agree that we can work well without it?
>> The point is to avoid diving into QEMU's code and instead use the current
>> out-of-the-box tracing points as the instrumentation hooks, which can be
>> achieved both through the current approach or an unofficial tracing backend.
>> The nice thing about the current approach is that the user can do some extra
>> checks on the tracing event and then (maybe) call the original 
>> backend-generated
>> tracing routine (as a quick and easy way to extend what to actually trace).

> Tracepoints shouldn't also suffer from bit rot so easily.

Exactly. That's why I thought that tracepoint instrumentation is not such a bad


 "And it's much the same thing with knowledge, for whenever you learn
 something new, the whole world becomes that much richer."
 -- The Princess of Pure Reason, as told by Norton Juster in The Phantom

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