Re: [RFC] [PATCH 1/7] User Space Breakpoint Assistance Layer (UBP)

From: Avi Kivity
Date: Sun Jan 17 2010 - 09:37:50 EST

On 01/16/2010 02:58 AM, Jim Keniston wrote:

I hear (er, read) you. Emulation may turn out to be the answer for some
architectures. But here are some things to keep in mind about the
various approaches:

1. Single-stepping inline is easiest: you need to know very little about
the instruction set you're probing. But it's inadequate for
multithreaded apps.
2. Single-stepping out of line solves the multithreading issue (as do #3
and #4), but requires more knowledge of the instruction set. (In
particular, calls, jumps, and returns need special care; as do
rip-relative instructions in x86_64.) I count 9 architectures that
support kprobes. I think most of these do SSOL.
3. "Boosted" probes (where an appended jump instruction removes the need
for the single-step trap on many instructions) require even more
knowledge of the instruction set, and like SSOL, require XOL slots.
Right now, as far as I know, x86 is the only architecture with boosted
4. Emulation removes the need for the XOL area, but requires pretty much
total knowledge of the instruction set. It's also a performance win for
architectures that can't do #3. I see kvm implemented on 4
architectures (ia64, powerpc, s390, x86). Coincidentally, those are the
architectures to which uprobes (old uprobes, with ubp and xol bundled
in) has already been ported (though Intel hasn't been maintaining their
ia64 port). So it sort of comes down to how objectionable the XOL vma
(or page) really is.

The kvm emulator emulates only a subset of the x86 instruction set (basically mmio instructions and commonly-used page-table manipulation instructions, as well as some privileged instructions). It would take a lot of work to expand it to be completely generic; and even then it will fail if userspace uses an instruction set extension the kernel is not aware of.

To me, boosted probes with a fallback to single-stepping seems to be the better option by far.

error compiling committee.c: too many arguments to function

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