Re: [Qemu-devel] [PATCH 4/4] target-arm: always set endian bits in big-endian mode

[Fabien Chouteau]

On 03/04/2013 02:24 PM, Paul Brook wrote:
>> On 03/01/2013 09:58 PM, Paul Brook wrote:
>>>> +#ifdef TARGET_WORDS_BIGENDIAN
>>>> +    if (arm_feature(env, ARM_FEATURE_V6)
>>>> +        || arm_feature(env, ARM_FEATURE_V7)) {
>>>> +        /* IE and EE bits stay set for big-endian */
>>>> +        env->cp15.c1_sys |= (1 << 31) | (1 << 25);
>>>> +    }
>>>> +#endif
>>>
>>> This is wrong for all the CPUs QEMU crrently supports. SCTLR.IE is
>>> defined to be zero.
>>
>> Again I'd like to have more information. Why is it wrong to set IE when
>> we are in big-endian?
>
> The ARM architecture defines two big-endian modes.  In BE8 mode only data 
> accesses big-endian, code fetches are still little-endian.  In BE32 mode both 
> code and data are big-endian.  In theory a fourth mode (big-endian code, 
> little-endian data) exists, though I've never seen that used.
>

I'm a bit lost. You say that BE32 means data and instruction in
big-endian and BE8 only data in big-endian. And this is confirmed by
Peter's article :
(http://translatedcode.wordpress.com/2012/04/02/this-end-up/).

For me there's two different things:
- big-endian kind: BE32 or BE8
- endianness of data/instruction

Is it possible to have both data and instruction in BE8?

Now in the ARMv7 ARM chapter A3.3.2:

"Instruction endianness static configuration, ARMv7-R only

To provide support for legacy big-endian object code, the ARMv7-R
profile supports optional byte order reversal hardware as a static
option from reset. The ARMv7-R profile includes a read-only bit in the
CP15 Control Register, SCTLR.IE, bit [31]. For more information, see c1,
System Control Register (SCTLR) on page B4-45."

Since it is a legacy support, I would imagine that SCTLR.IE means BE32
access for instructions. Is that right?

> All the v7 cores QEMU currently supports[1] only implement BE8 mode.  The IE 
> bit is reserved and most be zero.  Usermode emulation implements both, but 
> the 
> privileged cp15 registers can safely be ignored there.
>

When I build my qemu-system-armeb, in what mode is it (BE8, BE32, data
and/or instruction in big-endian)?

Thanks,

-- 
Fabien Chouteau