Re: [Qemu-devel] [PATCH] PXA-specific ARM hacks.

[Paul Brook <paul@codesourcery.com>]

On Friday 16 March 2007 22:06, andrzej zaborowski wrote:
> This patch contains some ugly but very useful chnages:
>  - in cpu-exec.c, halt the arm CPU on CPU_INTERRUPT_HALT like in other
> architectures.

This is ok. Though I'd prefer it to actually be common code (like 
CPU_INTERRUPT_DEBUG).

>  - wake the cpu up on CPU_INTERRUPT_EXITTB - this serves waking the
> CPU up without asserting IRQ or FIQ, which is possible on PXA. Would
> it be better to add a separate interrupt type instead?

I think this is probably ok. Could do with a comment somewhere saying so.

>  - in target-arm/translate.c, don't terminate the TB after a CP15
> write. The purpose of this is to imitate the real processor's
> instruction cache (to some degree). OSes should never need rely on the
> cache prefetch but unfortunately Linux for pxa does when it's waking
> up from sleep or deep idle, when enabling the MMU (simplifies things a
> lot for Linux).

This is not acceptable in its current form. IIRC on arm cores the effects take 
effect once all insn have worked through the pipeline (typically a few 
cycles), and I bet there's code that relies on this. ie:
mcr cp15, ...;nop;nop;nop;nop;@expect cp15 write to have taken effect by now

A more acceptable solution would be:
- Only extend the TB for the specific instruction that linux abuses
- Limit the number of instructions that can follow before the end of the TB.
- Add a comment saying what this hack is for.

Paul