linux-kernel - Re: [PATCH 3/4] x86: Rewrite switch

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Message-ID: <20160712141628.qlo2lhhjiyxfs6sw@treble>
Date:	Tue, 12 Jul 2016 09:16:28 -0500
From:	Josh Poimboeuf <jpoimboe@...hat.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Brian Gerst <brgerst@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Denys Vlasenko <dvlasenk@...hat.com>,
	Ingo Molnar <mingo@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...e.de>, "H. Peter Anvin" <hpa@...or.com>,
	X86 ML <x86@...nel.org>, Dave Anderson <anderson@...hat.com>
Subject: Re: [PATCH 3/4] x86: Rewrite switch_to() code

On Mon, May 23, 2016 at 10:03:54AM -0700, Andy Lutomirski wrote:
> Do you have any understanding of why there are so many unwinder
> implementations?  Your reliable unwinder seems to be yet another copy
> of more or less the same code.
> 
> I'd like to see a single, high-quality unwinder implemented as a state
> machine, along the lines of:
> 
> struct unwind_state state;
> unwind_start_inactive_task(&state, ...); or
> unwind_start_pt_regs(&state, regs); or whatever.
> unwind_next_frame(&state);
> 
> where, after unwind_next_frame, state encodes whatever registers are
> known (at least bp and ip, but all the GPRs would be nice and are
> probably mandatory for DWARF) and an indication of whether this is a
> real frame or a guessed frame (the things that currently show up as
> '?').

FYI, I'm working on something very similar to this which replaces
dump_trace().  The frame pointer encoding patches were going to require
more changes to the unwinder than I expected, and more callback sprawl.
So it looks like it's going to be easier to just go ahead and rewrite
the unwinder first.

-- 
Josh