Re: [Qemu-devel] [PATCH v7 00/42] Invert Endian bit in SPARCv9 MMU TTE

[<tony.nguyen@bt.com>]

Hi Phillippe,

On 8/16/19 7:58 PM, Philippe Mathieu-Daudé wrote:
>On 8/16/19 8:28 AM, tony.nguyen@bt.com wrote:
>> This patchset implements the IE (Invert Endian) bit in SPARCv9 MMU TTE.
>>
>> v7:
>[...]
>> - Re-declared many native endian devices as little or big endian. This is why
>>   v7 has +16 patches.
>
>Why are you doing that? What is the rational?

While collapsing the byte swaps, it was suggested in patch #11 of v5 that
consistent use of MemOp simplified endian comparisons. This lead to the
deprecation of enum device_endian by MemOp.

As MO_TE is conditional upon NEED_CPU_H, the s/DEVICE_NATIVE_ENDIAN/MO_TE/
required changing some device object files from common-obj-* to obj-*. In patch
#15 of v6 Paolo noted that most devices should not of been DEVICE_NATIVE_ENDIAN
and hinted at a clean up.

The +16 patches in v7 is the clean up effort.

>Anyhow if this not required by your series, you should split it out of
>it, and send it on your principal changes are merged.
>I'm worried because this these new patches involve many subsystems (thus
>maintainers) and reviewing them will now take a fair amount of time.

Yes, lets split these patches out. They are very much a tangent to the series
purpose.

>> For each device declared with DEVICE_NATIVE_ENDIAN, find the set of
>> targets from the set of target/hw/*/device.o.
>>
>> If the set of targets are all little or all big endian, re-declare
>> the device endianness as DEVICE_LITTLE_ENDIAN or DEVICE_BIG_ENDIAN
>> respectively.
>
>If only little endian targets use a device, that doesn't mean the device
>is designed in little endian...
>
>Then if a big endian target plan to use this device, it will require
>more work and you might have introduced regressions...
>
>I'm not sure this is a safe move.
>
>> This *naive* deduction may result in genuinely native endian devices
>> being incorrectly declared as little or big endian, but should not
>> introduce regressions for current targets.
>

Roger. Evidently too naive. TBH, most devices I've never heard of...

Regards,
Tony