Re: [PATCH -tip 0/4 V3] tracing: kprobe-based event tracer

Masami Hiramatsu
2009-04-01T23:29:54Z

Andi Kleen wrote:
> On Wed, Apr 01, 2009 at 04:51:00PM -0400, Masami Hiramatsu wrote:
>> Andi Kleen wrote:
>>> Masami Hiramatsu <mhiramat@redhat.com> writes:
>>>> I agreed. Fortunately, Jim Keniston and I wrote an x86 instruction
>>>> decoder :-) which has been made originally for uprobe andd kprobes
>>>> jump-optimizer.
>>>>
>>>> https://www.redhat.com/archives/utrace-devel/2009-March/msg00031.html
>>> An alternative would be to adapt the x86 interpreter in KVM.
>>> I thought for some time that that one should be available in
>>> a more generic form in a library.
>> As far as I can see, KVM's instruction emulator is incomplete
>
> That's fine for you -- you only care about a subset of instructions
> anyways, don't you?

Actually, (in my case) I just need to decode non-FPU instructions,
because I'd like to check whether kprobe is on the instruction
boundary.

However, KVM's insn decoder can't decode some elemental
instructions, and instruction flags are incorrect.
I had written instruction decoder based on it, but the result
was so awful!
So soon, I had to rewrite it based on Intel's manual entirely :-(

>> (it doesn't cover all instructions...) and aims to emulate
>> instructions, not to analyze (so I couldn't relay on it).
>
> You can use it to analyze, just plug in the right callbacks that
> do nothing. I looked at it some time ago for doing instruction
> length checking for some application, but that application
> then disappeared. The main obstacle with making it a library
> is that some KVM specific dependencies have crept in that would
> need to be abstracted again, but I don't think it would need a lot of
> effort,

Sorry, but I don't think so. Current KVM's decoder is much more
focusing on preparing instructions emulation. It requires
vcpu setup, fetching operators and so on. I think it needs to
diet their code (or well splitting from emulator).

Anyway, I don't stick with my decoder. If they can provide more
generic interfaces, I'd be happy to use it. :-)

Thank you,