Re: [Qemu-devel] [PATCH] fix ARMv7 MOV R15, xxx operation

[Riihimaki Juha <juha.riihimaki@nokia.com>]

On Mar 13, 2009, at 17:13, ext Laurent Desnogues wrote:

> On Fri, Mar 13, 2009 at 3:18 PM, Riihimaki Juha
> <Juha.Riihimaki@nokia.com> wrote:
>> I suppose
>> changing the store_reg and gen_movl_reg_TN functions in
>> target-arm/translate.c to include an extra check when destination  
>> is R15
>> like "if in ARM state and arch >= 7 then bx else..." might do the  
>> trick more
>> generally but do you think it introduces the change in a too wide  
>> scope?
>
> I'm afraid that'd be too wide as it would effect ld/st exclusive,
> gen_exception_return and gen_lookup_tb (plus iwmmxt code).
>

Thanks, that is what I thought as well. It would also have slowed down  
thumb processing due to the extra check introduced in all stores to  
r15 even though the changed functionality only affects arm state. I  
guess then that there is no generic place where this could be easily  
fixed but instead the handling of all affected commands would need to  
be changed similarly to the patch that I sent for the mov command?  
Seems a little bit clumsy approach to duplicate the same piece of code  
in several places imho so if you have any better suggestions...

On a sidenote, I also noticed that the arm emulation currently allows  
jumping to unaligned memory addresses in arm mode since it only clears  
the least significant bit when storing to r15. However, since armv6  
the two least significant bits should be automatically ignored. This  
hasn't caused me any problems (yet) since I guess programs rarely rely  
on this feature.


Juha