linux-kernel - Re: [PATCHv4] arm: ftrace: Adds support for CONFIG_DYNAMIC_FTRACE_WITH

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Date:   Thu, 2 Mar 2017 23:03:32 +0000
From:   Russell King - ARM Linux <linux@...linux.org.uk>
To:     Abel Vesa <abelvesa@...il.com>
Cc:     Nicolai Stange <nicstange@...il.com>,
        Abel Vesa <abelvesa@...ux.com>, robin.murphy@....com,
        jjhiblot@...phandler.com, Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, pmladek@...e.com,
        mhiramat@...nel.org, linux-arm-kernel@...ts.infradead.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCHv4] arm: ftrace: Adds support for
 CONFIG_DYNAMIC_FTRACE_WITH_REGS

On Thu, Mar 02, 2017 at 09:01:24PM +0000, Abel Vesa wrote:
> On Tue, Feb 28, 2017 at 11:54:29AM +0000, Abel Vesa wrote:
> > On Tue, Feb 28, 2017 at 11:46:38AM +0000, Russell King - ARM Linux wrote:
> > > On Tue, Feb 28, 2017 at 11:22:27AM +0000, Abel Vesa wrote:
> > > > On Tue, Feb 28, 2017 at 11:58:49AM +0100, Nicolai Stange wrote:
> > > > > Hi Abel,
> > > > > 
> > > > > On Tue, Feb 28 2017, Abel Vesa wrote:
> > > > > 
> > > > > > On Mon, Feb 27, 2017 at 04:52:06PM +0100, Nicolai Stange wrote:
> > > > > >> On Fri, Feb 24 2017, Abel Vesa wrote:
> > > > > >> Wouldn't it be better (and more consistent with other archs) to have
> > > > > >> 
> > > > > >>   pt_regs->ARM_lr = original lr
> > > > > >>   pt_refs->ARM_pc = current lr
> > > > > >> 
> > > > > >> instead?
> > > > > 
> > > > > The stack would look like this then
> > > > > 
> > > > > @ ...           | ARM_ip | ARM_sp | ARM_lr      | ARM_pc      | ...          |
> > > > > @ 0  4          48       52       56            60            64    68       72
> > > > > @ R0 | R1 | ... | LR     | SP + 4 | original LR | original PC | PSR | OLD_R0 | original LR |
> Just to make sure we're on the same page. If we are replacing the LR 
> with the original_LR is it worth keeping around the one pushed before 
> the ftrace_regs_caller is called?
> 
> Another thing, PC needs to be new_LR and then we can restore all 
> regs r0 through r15 like this:
> 
> 	ldmia   sp, {r0-r15}

That's the intention - the point is to save the state as it was at the
point that the function was entered, not at the point when the ftrace
code was entered.  What we don't want is the implementation details of
GCC's mcount or ftrace's adaption of mcount being exposed via ftrace.

-- 
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