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

[Avi Kivity <avi@redhat.com>]

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
> kprobes.
> 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.

-- 
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