Re: [PATCH 1/2] perf: Move task_pt_regs sampling into arch code
From: Peter Zijlstra
Date: Mon Jan 05 2015 - 11:45:22 EST
On Mon, Jan 05, 2015 at 08:13:49AM -0800, Andy Lutomirski wrote:
> > Just wondering how bad it would be to fill out the actual pt_regs that
> > was previously partially initialized?
>
> Bad, for at least two reasons.
>
> One is that we don't actually know which regs are initialized. bx,
> bp, r12 etc are particularly bad in this regard, due to the FORK_LIKE
> mechanism and similar optimizations.
Right, but you need to deal with that anyhow.
> The other is that the uninitialized part of task_pt_regs can be used
> for something else entirely. If we have a syscall instruction
> immediately followed by a regular interrupt, then the interrupt's
> hardware frame will overlap task_pt_regs. (I'm not going to claim
> that this design is sensible, but it is what it is. IIRC Denys
> Vlasenko had some patches to partially clean this up.)
Ah, urgh. Yes painful that.
> It would be possible to rework the code to avoid an extra pt_regs
> copy, but I don't see an obvious way to do it cleanly.
Yeah, we'll see how this works, I was just curious on the exact need for
the copy, but if as you say, the original structure might not even exist
properly (even though we have a pointer to it) that's bad (TM).
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