linux-kernel - Re: [PATCH 15/20] ARM/hw_breakpoint: Convert to hotplug state machine

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Date:   Fri, 18 Nov 2016 14:59:04 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Will Deacon <will.deacon@....com>
cc:     Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        linux-kernel@...r.kernel.org, rt@...uxtronix.de,
        Mark Rutland <mark.rutland@....com>,
        Russell King <linux@...linux.org.uk>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 15/20] ARM/hw_breakpoint: Convert to hotplug state
 machine

On Fri, 18 Nov 2016, Will Deacon wrote:
> On Fri, Nov 18, 2016 at 02:42:15PM +0100, Thomas Gleixner wrote:
> > On Fri, 18 Nov 2016, Will Deacon wrote:
> > > On Fri, Nov 18, 2016 at 02:11:58PM +0100, Thomas Gleixner wrote:
> > > > But it's guaranteed that cpuhp_setup_state() will not return before the
> > > > callback has been invoked on each online cpu.
> > > 
> > > Ok, that's good.
> > > 
> > > > If cpus are not yet online when that code is invoked, then it's the same
> > > > behaviour as before. It will be invoked when the cpu comes online.
> > > 
> > > Just to check, but what stops a CPU from coming online between the call
> > > to cpuhp_setup_state and the call to cpuhp_remove_state_nocalls in the
> > > case of failure (debug_err_mask isn't empty)?
> > 
> > Indeed! I missed that part. So we still need a get/put_online_cpus()
> > protection around all of this.
> 
> Yes, that should do it.
> 
> > Just for curiosity sake. Wouldn't it be simpler and less error prone to
> > make the ARM_DBG_READ/WRITE macros use the exception table and handle that
> > in the undefined instruction handler to avoid this hook dance?
> 
> That would be an option, but it's only the reset sequence that could
> generate this fault so it's simpler to isolate it there. 

ARM_DBG_READ/WRITE_SAFE() then for reset_ctrl_regs()

> We'd also have to take into account SMP if we toggle the handler in the
> READ/WRITE accessors, since the fault handler framework is system-wide as
> opposed to per-cpu. The whole thing is grotty as hell.

The exception table is not toggling anything. It's just providing an entry
in the exception tables, which is scanned by fixup_exception(), which then
moves PC to the exception code. See __get_user_asm().

So the whole thing becomes:

static int reset_ctrl_regs(unsigned cpu)
{
	....
	if (ARM_DBG_READ_SAFE(c1, c5, 4, val))
		return -ENODEV;
	....
	return 0;
}

All you need is the extra

    	if (fixup_exception(regs))
		return;

in do_undefinstr() like it is there in do_kernel_fault(). No hooks, no
scope issues, just works.

I just mention this because that's how x86 implements rdmsr/wrmsr_safe() so
it can probe msr access. The difference though it that this results in a
#GP and not in #UD, but that's not a show stopper :)

Thanks,

	tglx