Simply disabling MMU in cpu_v7_reset doesn't work for kexec

[jamie@shareable.org (Jamie Lokier)]

Russell King - ARM Linux wrote:
> On Wed, Apr 21, 2010 at 09:19:06PM +0800, Eric Miao wrote:
> > On Wed, Apr 21, 2010 at 9:18 PM, Eric Miao <eric.y.miao@gmail.com> wrote:
> > > Tried the following simple code to disable MMU in cpu_v7_reset() doesn't
> > > seem to work for kexec reboot. I was expecting the following instructions
> > > after MMU is disabled are still reachable in the instruction fetching pipe.
> > >
> > 
> > 	.align	5
> >  ENTRY(cpu_v7_reset)
> > +	mrc	p15, 0, r1, c1, c0, 0		@ ctrl register
> > +	bic	r1, r1, #0x0001			@ ...............m
> > +	mcr	p15, 0, r1, c1, c0, 0		@ disable MMU
> > +	mcr	p15, 0, ip, c8, c7, 0		@ invalidate I and D TLBs
> >  	mov	pc, r0
> >  ENDPROC(cpu_v7_reset)
> 
> The procedure expected here is:
> 
> 1. invalidate caches
> 2. drain buffers
> 3. invalidate TLBs
> 4. disable MMU et.al.
> 5. jump to the address in r0.
> 
> The reason this is permitted is because cpu_v7_proc_fin _must_ be called
> before hand, which is expected to disable all interrupts, and flush and
> disable the caches.

Isn't step 1 above redundant, because caches are already disabled and
flushed?

> It might be worth given the speculative behaviour of V7 to also invalidate
> TLBs after a dsb and isb after step 4 instead of before step 4 - and (iirc)
> also doing an isb after invalidating the TLBs.

Mightn't the CPU refill one entry in the TLB between steps 3 and 4,
just by fetching the instructions - even non-speculatively?

Fwiw, the lengthy patch here:

   http://lists.infradead.org/pipermail/barebox/2010-January/000528.html

does exactly this in the ARMv7 part:

1. Turn off MMU and caches (one mcr).
2. Clean and flush caches.
3. Invalidate TLBs.
4. Invalidate BTC.
5. DSB.
6. ISB.
7. Return to caller / destination.

I don't know if it expects the final instructions to be direct-mapped.

-- Jamie