[PATCH v5 3/8] ARM: idmap: populate identity map pgd at init time

[Russell King - ARM Linux]

On Tue, Nov 08, 2011 at 03:52:58PM +0000, Will Deacon wrote:
> When disabling the MMU, it is necessary to take out an identity mapping
> for as much of the address space as possible so that the reset path can
> be executed using its physical address. This is useful for softbooting
> and entering low power states.
> 
> This patch allocates a set of pagetables during boot and populates them
> with an identity mapping for all of memory apart from a small window
> containing the kernel image.

I'd like to suggest a slightly different approach, now that we have the
'soft_restart()' thing which is guaranteed to be handling all of the
soft-restart stuff.

First, what does the soft-restart code need to do?  It needs to:
1. Perform an orderly shutdown of caches.
2. Perform an orderly shutdown of the MMU.
3. Jump to the supplied physical address with the caches and MMU off,
   IRQs disabled, etc.

And for (3) to work across the range of processors with ease, we need
the code to run in a 1:1 mapping so turning off the MMU doesn't leads
to unpredictable results.

We can achieve most of this in the way that we're already doing, so:
1. calling local_irq_disable() and local_fiq_disable() to disable interrupts
2. cpu_proc_fin() to disable caches
3. flush_cache_all() (and outer_flush_all()).

So far, that's very much what the new soft_restart() function already
does.  But - can we be smarter about the 1:1 mapping like we are with
the CPU suspend code?  Or, to put it another way, can we use the CPU
suspend code's page table which it allocated, and setup a mapping to
cover just the cpu_reset() code itself.

We'd need to reorganize the cpu_reset() code to ensure that the MMU
is actually turned off before we do the final jump, but that shouldn't
be too hard to achieve.

In other words, what I'm saying is that we don't need a 1:1 mapping
setup for the entire userland _if_ we're careful enough with how we
deal with the MMU-off -> jump transition.

This should fix the concerns people have had with kexec and kdump, as
well as fixing up the (broken) v6 and v7 cpu_reset() code.