linux-kernel - Re: Invalid perf_branch_entry.to entries question

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Message-ID: <20130510104352.GF31235@dyad.programming.kicks-ass.net>
Date:	Fri, 10 May 2013 12:43:52 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Michael Neuling <mikey@...ling.org>
Cc:	eranian@...gle.com, Linux PPC dev <linuxppc-dev@...abs.org>,
	linux-kernel@...r.kernel.org, michael@...erman.id.au,
	Anshuman Khandual <anshuman.khandual@...ibm.com>
Subject: Re: Invalid perf_branch_entry.to entries question

On Thu, May 09, 2013 at 08:39:15AM +1000, Michael Neuling wrote:
> > Just because I'm curious.. however does that happen? Surely the CPU
> > knows where next to fetch instructions?
> 
> For computed gotos (ie. branch to a register value), the hardware gives
> you the from and to address in the branch history buffer.
> 
> For branches where the branch target address is an immediate encoded in
> the instruction, the hardware only logs the from address.  It assumes
> that software (perf irq handler in this case) can read this branch
> instruction, calculate the corresponding offset and hence the
> to/target address.
> 
> It's entirely possible that when the perf IRQ handler happens, the
> instruction in question is not readable or is no longer a branch (self
> modifying code).  Hence we aren't able to calculate a valid to address.

Ohh how cute! You've gotta love lazy hardware :-)
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