linux-kernel - Re: [PATCH 1/3] nmi: create generic NMI backtrace implementation

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Message-ID: <20150725144229.GZ7557@n2100.arm.linux.org.uk>
Date:	Sat, 25 Jul 2015 15:42:29 +0100
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Daniel Thompson <daniel.thompson@...aro.org>
Cc:	linux-arm-kernel@...ts.infradead.org, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/3] nmi: create generic NMI backtrace implementation

On Thu, Jul 16, 2015 at 10:51:25AM +0100, Daniel Thompson wrote:
> On 16/07/15 10:37, Russell King - ARM Linux wrote:
> >That can be implemented in the arch raise() method if needed - most
> >architectures shouldn't need it as if they are properly raising a NMI
> >which is, by definition, deliverable with normal IRQs disabled.
> 
> Agreed. The bug certainly could be fixed in the ARM raise() function.
> 
> However I'm still curious whether there is any architecture that benefits
> from forcing the current CPU into an NMI handler? Why doesn't the
> don't-run-unnecessary-code argument apply here as well?

The benefit is that we get a consistent way of invoking the backtrace,
since causing the NMI exception gives us a 'struct pt_regs' to work
with, which we wouldn't otherwise have if we tried to call it "inline".

The NMI backtrace includes dumping the register state of the NMI-
receiving CPUs, which needs a 'struct pt_regs' and generating a that in
arch-independent code wouldn't be nice.

In any case, if this area needs changing in the generic code, it should
be done as a separate change so that it can be properly assessed and
validated on x86.

In the mean time, I will action Thomas' request to put it into my tree
so that we can get some reasonable linux-next time with it, and hopefully
have some progress towards FIQ-based backtracing for ARM.

Thanks.

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