linux-kernel - Re: [PATCH 7/7] DWARF: add the config option

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Message-ID: <20170522111248.q74jgpqlwxl5oby7@gmail.com>
Date:   Mon, 22 May 2017 13:12:48 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Josh Poimboeuf <jpoimboe@...hat.com>
Cc:     "H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
        Jiri Slaby <jslaby@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        live-patching@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 7/7] DWARF: add the config option


* Josh Poimboeuf <jpoimboe@...hat.com> wrote:

> > How are you handling control flow?
> 
> Control flow of what?
> 
> > > Here's the struct in its current state:
> > > 
> > > 	#define UNDWARF_REG_UNDEFINED		0
> > > 	#define UNDWARF_REG_CFA			1
> > > 	#define UNDWARF_REG_SP			2
> > > 	#define UNDWARF_REG_FP			3
> > > 	#define UNDWARF_REG_SP_INDIRECT		4
> > > 	#define UNDWARF_REG_FP_INDIRECT		5
> > > 	#define UNDWARF_REG_R10			6
> > > 	#define UNDWARF_REG_DI			7
> > > 	#define UNDWARF_REG_DX			8
> > > 
> > 
> > Why only those registers?  Also, if you have the option I would really
> > suggest using the actual x86 register numbers (ax, ex, dx, bx, sp, bp,
> > si, di, r8-r15 in that order.)
> 
> Those are the only registers which are ever needed as the base for
> finding the previous stack frame.  99% of the time it's sp or bp, the
> other registers are needed for aligned stacks and entry code.
> 
> Using the actual register numbers isn't an option because I don't need
> them all and they need to fit in a small number of bits.
> 
> This construct might be useful for other arches, which is why I called
> it "FP" instead of "BP".  But then I ruined that with the last 3 :-)

Please call it BP - 'FP' can easily be read as floating-point, making it all 
super-confusing. We should use canonical x86 register names and ordering - even
if not all registers are used straight away.

Thanks,

	Ingo