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

From: Ananth N Mavinakayanahalli
Date: Mon Jan 18 2010 - 12:11:19 EST

On Mon, Jan 18, 2010 at 06:52:32PM +0200, Avi Kivity wrote:
> On 01/18/2010 05:43 PM, Ananth N Mavinakayanahalli wrote:
>>>> Well, the alternatives are very unappealing. Emulation and single-stepping
>>>> are going to be very slow compared to a couple of jumps.
>>> So how big chunks of the address space are we talking here for uprobes?
>> As Srikar mentioned, the least we start with is 1 page. Though you can
>> have as many probes as you want, there are certain optimizations we can
>> do, depending on the most common usecases.
>> For eg., if you'd consider the start of a routine to be the most
>> commonly traced location, most routines in a binary would generally
>> start with the same instruction (say push %ebp), and we can refcount a
>> slot with that instruction to be used for all probes of the same
>> instruction.
> But then you can't follow the instruction with a jump back to the code...

Right. This will work only for the non boosted case where single-stepping
is mandatory. I guess the tradeoff is vma space and speed.

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