Re: [PATCH 2/2] arm: apply more __ro_after_init

[Daniel Thompson]

On 11/08/16 17:02, Arnd Bergmann wrote:
> On Thursday, August 11, 2016 12:02:42 AM CEST Russell King - ARM Linux wrote:
> > On Wed, Aug 10, 2016 at 09:31:05PM +0200, Arnd Bergmann wrote:
> > > On Wednesday, August 10, 2016 11:12:53 AM CEST Russell King - ARM Linux wrote:
> > > > There's the TLS emulation too, but that writes via the vectors mapping
> > > > at 0xffff0ff0.
> > >
> > > Ok, so that should be safe. Can we change the fiq code to also use the
> > > high mapping and then take the __ro_after_init patch on top?
> >
> > We can't - if the kernel is configured without the kuser helpers in
> > the vectors page, it's mapped read-only. I'm not sure what the
> > intersection is between platforms that can have FIQs and platforms
> > that can disable the kuser helpers.
>
> From Kconfig logic and callers of set_fiq_handler(), theoretically
> there is just i.MX3, but I think they never use fiq in their
> audio drivers in practice already, and Mark Brown mentioned
> that we could remove fiq support in the imx audio driver (don't
> remember the details at the moment).
>
> If we can prove that i.MX3 PCM FIQ support is never used, then the
> intersection is empty, and all machines that use FIQ require kuser
> helpers.
>
> This may change with Daniel Thompson's patches that use the FIQ
> for NMI backtrace.

It shouldn't do!

All the work I did (and am, very slowly, still doing) worked by using 
the default FIQ handler provided at boot time to jump into the perf code.

Nothing I have done or plan to do needs set_fiq_handler() to remain 
functional.

Likewise, nothing I have done should cause set_fiq_handler() to stop 
working for people who do still use it. FWIW I got the impression over 
the last few years that the most significant uses of FIQ on modern 
systems are out-of-tree uses who have designed custom FPGA hardware (and 
presumably designed them with very short FIFOs).


Daniel.