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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: Wed, 07 Dec 2011 23:03:50 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.0.92 (gnu/linux)

Lluís Vilanova writes:
> From my experience, these are the basics I need:

> * Decide what to do when an event is translated (by default - no 
> instrumentation
>   -, it just generates a TCG call to an execution-time tracing routine).

>   This includes:

>   * Deciding whether to generate the TCG call to the execution-time tracing
>     routine.

>   * Access to generating other arbitrary TCG code (the API is pretty stable, 
> but
>     headers depend on target-specific defines).

>     This could also include generating code like incrementing a counter (e.g.,
>     when counting instructions), instead of calling the execution-time tracing
>     routine.

>     My library actually hides this from the user, who just provides conditions
>     to establish whether to trace the event. Then the library automatically
>     establishes whether it's best to evaluate each condition at translation
>     time, using TCG code or when the execution-time tracing routine is 
> invoked.

>   * [still don't have it] Access to a map between target-specific abstractions
>     (mainly register names) and TCG values (e.g., to generate code that gets 
> or
>     sets the value of a well-known register).

> * Decide what to do when an event is executed, including calls to 
> execution-time
>   event tracing routines generated by TCG code (by default, it just calls the
>   tracing backend).

>   This includes:

>   * Arbitrary user-provided code (the analysis itself when not inlined with
>     TCG).

>   * [still don't have it] Access to get and set the values of target-specific
>     abstractions (mainly registers).

> * Add a per-vCPU opaque pointer with data private to the analyzer code.

I forgot to add that another nice addition is to let the translation-time
user-code specify which global registers might be accessed in the execution-time
user code (thus the translation can generate TCG_CALL_CONST by default).

Right now, it is established at compile time by having two event properties; one
with and one without TCG_CALL_CONST-enabled TCG helpers.


 "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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