[Qemu-devel] Trace ARM PC

[Qemu-devel] Trace ARM PC

[Xin Tong]

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Hi.

I would like to trace all the executed instruction PC in QEMU ARM. Because
ARM has conditional execution, we do not know whether an instruction will
execute or not at translation time. Therefore the PC tracing code can not
be generated before the instruction is disassembled. (i.e. before
disas_thumb_insn/disas_arm_insn ). Then, is it correct to generate the PC
tracing code after the disas_XXX_insn  functions are called ? I can keep
the old value of the PC before the PC in the disassemble context is updated
by the disas_XXX_insn.

I think this would work for normal instructions, but probably not for
branches, so the PC tracing has to be done before the branch in the
disas_XXX_insn functions ? can anyone please confirm ?

Thank you,
Xin

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Re: [Qemu-devel] Trace ARM PC

[Peter Maydell]

On 3 November 2013 10:31, Xin Tong <trent.tong@gmail.com> wrote:
> I would like to trace all the executed instruction PC in QEMU ARM. Because
> ARM has conditional execution, we do not know whether an instruction will
> execute or not at translation time.

This is a rather odd way to look at conditional execution. It's better
to think of it as "the instruction executes but may execute as a NOP".
(In particular, it's architecturally valid for an instruction which fails
its condition check to take an UNDEF exception, though QEMU
chooses not to do this.)

> Therefore the PC tracing code can not be
> generated before the instruction is disassembled. (i.e. before
> disas_thumb_insn/disas_arm_insn ). Then, is it correct to generate the PC
> tracing code after the disas_XXX_insn  functions are called ? I can keep the
> old value of the PC before the PC in the disassemble context is updated by
> the disas_XXX_insn.

The more usual way to trace this kind of thing if you care about
ccpass/ccfail status would be to always trace the PC of the
instruction about to be executed, and then also trace ccpass/ccfail
indications when the instruction is conditional.

-- PMM