Re: [Qemu-devel] Register usage statistics?

[Alexander Graf]

On 26.08.2011, at 16:52, Benjamin Ylvisaker wrote:

> On Fri, 26 Aug 2011 19:35 +0200, "Lluís" <address@hidden> wrote:
>> Benjamin Ylvisaker writes:
>> 
>>> I am working on a project for which I'd like to gather statistics
>>> about register usage.  (For example, distributions of degree-of-use
>>> and dynamic lifetimes of register-values.)  Is QEMU a good tool for
>>> doing this kind of work?  Has anyone else done this?  I looked at the
>>> tracing available in QEMU and it seems to be mostly for higher level
>>> things like memory accesses, function calls, basic block executions,
>>> etc. 
>> 
>> This is not currently possible.
>> 
>> In fact, I've been developing (somewhat in the background) the kind of
>> features you're asking for, but it will still be at least a couple of
>> months before it's in a publishable state.
>> 
>> Meanwhile, there are some tools you can use to accomplish this
>> (depending on your requirements), like valgrind or pintool.
> 
> By "not currently possible" do you mean the current tracing facilities
> don't support register/instruction granularity, or that there's
> something in the QEMU architecture itself that would require substantial
> changes to gather such information?  I am willing to put in a fair bit
> of hacking effort.  I only have a cursory understanding of the internals
> of QEMU, but shouldn't it be possible to hack the instruction
> translations to do some extra work?
> 
> I had been under the impression that QEMU was more likely to do what I
> want than Valgrind or Pin.  I thought those tools were a bit higher
> level and had no intention of giving instruction-granularity control of
> the guest program.  I will take another look, though.

If all you need are register access counters, you can simply have tcg increment 
a var whenever a register is used. Obviously, you'd have to go through all the 
code to make sure that it uses your wrapper, but I don't see why it wouldn't 
work :)

Valgrind is a JIT as well, so putting it in there would work as well as in QEMU.


Alex