Re: [Qemu-devel] [RFC] Tracing guest register usage

[Lluís Vilanova]

Richard Henderson writes:

> On 09/30/2016 08:13 AM, Lluís Vilanova wrote:
>>> (2) an internal state change
>>> to DisasContext, reflected in INDEX_op_insn_start, with no changes to any 
>>> TCG
>>> registers.  So you'll not actually be tracking eflags at all.
>> 
>> I don't follow what you mean. AFAIK INDEX_op_insn_start does not change the
>> guest eflags.

> It sets cc_op, which affects how eflags is computed.

I see tcg_gen_insn_start() gets dc->cc_op as a second argument, but I really
don't see where it gets modified (I'm looking at tcg_gen_code() on the
INDEX_op_insn_start case).

If you have the time, I'd like to understand that; I'm just curious.

But regardless of this specific case, we still have all the instructions
implemented with TCG helpers, which won't have any reguster usage information. I
was pretty convinced that was enough for some basic analysis using the traces,
but I might just as well keep proper register usage on my instrumentation tree.

Thanks,
  Lluis