Re: [PATCH v2 26/30] hw/arm/armv7m: prepare compilation unit to be common

[Pierrick Bouvier <pierrick.bouvier@linaro.org>]

On 3/24/25 18:22, Richard Henderson wrote:
> On 3/24/25 14:31, Pierrick Bouvier wrote:
>> On 3/23/25 12:48, Richard Henderson wrote:
>>> On 3/20/25 15:29, Pierrick Bouvier wrote:
>>>> Signed-off-by: Pierrick Bouvier <pierrick.bouvier@linaro.org>
>>>> ---
>>>>     hw/arm/armv7m.c | 12 ++++++++----
>>>>     1 file changed, 8 insertions(+), 4 deletions(-)
>>>>
>>>> diff --git a/hw/arm/armv7m.c b/hw/arm/armv7m.c
>>>> index 98a69846119..c367c2dcb99 100644
>>>> --- a/hw/arm/armv7m.c
>>>> +++ b/hw/arm/armv7m.c
>>>> @@ -139,8 +139,9 @@ static MemTxResult v7m_sysreg_ns_write(void *opaque, hwaddr addr,
>>>>         if (attrs.secure) {
>>>>             /* S accesses to the alias act like NS accesses to the real region */
>>>>             attrs.secure = 0;
>>>> +        MemOp end = target_words_bigendian() ? MO_BE : MO_LE;
>>>>             return memory_region_dispatch_write(mr, addr, value,
>>>> -                                            size_memop(size) | MO_TE, attrs);
>>>> +                                            size_memop(size) | end, attrs);
>>>
>>> target_words_bigendian() is always false for arm system mode.
>>> Just s/TE/LE/.
>>>
>>
>> Good point.
>>
>> By the way, what's the QEMU rationale behind having Arm big endian user binaries, and not
>> provide it for softmmu binaries?
> 
> For system mode, endianness is set via a combination of CPSR.E, SCTLR.B and SCTLR.EE,
> details depending on armv4, armv6, armv7+.
> 
> It is IMPLEMENTATION DEFINED how the cpu initiailizes at reset.  In olden times, via a
> board-level pin (sometimes switched, sometimes soldered).  We model the board-level pin
> via the "cfgend" cpu property.
> 
> In any case, for system mode we expect the guest to do the same thing it would need to do
> on real hardware.  For user mode, we can't do that, as we're also emulating the OS layer,
> which needs to know the endianness to expect from the guest binaries.
> 

Oh yes, totally makes sense. Thanks.

>> If those systems are so rare, why would people need a user mode emulation?
> 
> IMO armbe-linux-user is extinct.
> 
> Debian never had big-endian support at all.  If there was some other distro which had it,
> I don't recall which.  Otherwise you'd need to bootstrap the entire toolchain, which to me
> seems rather beside the point.
> 
> 
> r~