Re: [PATCH v6 07/36] nds32: Exception handling

[Greentime Hu]

Hi, Arnd:

2018-01-30 21:33 GMT+08:00 Arnd Bergmann <arnd@xxxxxxxx>:
> On Tue, Jan 30, 2018 at 11:01 AM, Vincent Chen <deanbo422@xxxxxxxxx> wrote:
>> 2018-01-24 19:10 GMT+08:00 Arnd Bergmann <arnd@xxxxxxxx>:
>>> On Wed, Jan 24, 2018 at 12:09 PM, Arnd Bergmann <arnd@xxxxxxxx> wrote:
>>>> On Wed, Jan 24, 2018 at 11:53 AM, Vincent Chen <deanbo422@xxxxxxxxx> wrote:
>>>>> 2018-01-18 18:14 GMT+08:00 Arnd Bergmann <arnd@xxxxxxxx>:
>>>
>>>> Ok. I still wonder about the kernel part of this though: is it a good idea
>>>> for user space to configure whether the kernel does unaligned
>>>> accesses? I would think that the kernel should just be fixed in such
>>>> a case.
>>>
>>> To clarify: I'm asking only about unaligned accesses from kernel code itself,
>>> which is generally considered a bug when
>>> CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS is disabled.
>>>
>>>       Arnd
>>
>> Thanks for your comments.
>>
>> For performance, we decide always disable
>> CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS even if hardware supports
>> unaligned accessing. Therefore, I will remove kernel unaligned accessing from
>> nds32/mm/alignment.c. In other words, alignment.c only addresses unaligned
>> accessing for user space.
>
> I'm not really following that logic, let's go through that again so I understand
> the situation better.
>
> CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS should be set if and
> only if you have a CPU that does not need to trap on unaligned accesses.
>
> What are the hardware capabilities on nds32? Do you have all three
> categories:
>
> a) some CPUs that always trap on unaligned access
> b) some CPUs that never trap on unaligned access
> c) some CPUs that can be configured to either trap or not trap by
>     the kernel?
>
We have type a and c.
We use CONFIG_ALIGNMENT_TRAP for a and
CONFIG_HW_SUPPORT_UNALIGNMENT_ACCESS for c.

Since unaligned access in kernel code itself should be considered as a
bug, we will remove the emulation code to handle the kernel code
unaligned accessed case.
We think CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS and
CONFIG_HW_SUPPORT_UNALIGNMENT_ACCESS have different purposes because
it will still be more efficient to access by byte even if hardware
support unaligned access.
CONFIG_HW_SUPPORT_UNALIGNMENT_ACCESS is used to prevent generating
unaligned access exception.

Thus, we will
1. treat unaligned access in kernel code itself as a bug
2. don't select CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
3. disable CONFIG_HW_SUPPORT_UNALIGNMENT_ACCESS as default