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

[Avi Kivity]

On 01/19/2010 07:47 PM, Jim Keniston wrote:
> > This is still with a kernel entry, yes?
> >      
> Yes, this involves setting a breakpoint and trapping into the kernel
> when it's hit.  The 6-7x figure is with the current 2-trap approach
> (breakpoint, single-step).  Boosting could presumably make that more
> like 12-14x.
>    

A trap is IIRC ~1000 cycles, we can reduce this to ~50 (totally 
negligible from the executed code's point of view).

> > Do you have plans for a variant
> > that's completely in userspace?
> >      
> I don't know of any such plans, but I'd be interested to read more of
> your thoughts here.  As I understand it, you've suggested replacing the
> probed instruction with a jump into an instrumentation vma (the XOL
> area, or something similar).  Masami has demonstrated -- through his
> djprobes enhancement to kprobes -- that this can be done for many x86
> instructions.
>
> What does the code in the jumped-to vma do?

1. Write a trace entry into shared memory, trap into the kernel on overflow.
2. Trap if a condition is satisfied (fast watchpoint implementation).

> Is the instrumentation code
> that corresponds to the uprobe handlers encoded in an ad hoc .so?
>    

Looks like a good idea, but it doesn't matter much to me.

--
error compiling committee.c: too many arguments to function

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