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

[Vincent Chen]

2018-01-18 18:14 GMT+08:00 Arnd Bergmann <arnd@xxxxxxxx>:
> On Mon, Jan 15, 2018 at 6:53 AM, Greentime Hu <green.hu@xxxxxxxxx> wrote:
>> From: Greentime Hu <greentime@xxxxxxxxxxxxx>
>>
>> This patch includes the exception/interrupt entries, pt_reg structure and
>> related accessors.
>>
>> Signed-off-by: Vincent Chen <vincentc@xxxxxxxxxxxxx>
>> Signed-off-by: Greentime Hu <greentime@xxxxxxxxxxxxx>
>
> Here it would be good to have a more detailed explanation about the alignment
> trap handling. I remember discussing it with you before, but don't remember
> the exact outcome. In particular you should explain here why you need to
> handle alignment traps in the first place, and what the expected defaults
> are (e.g. always disabled unless a user requests it, or always enabled) and
> what kind of code runs into the traps (e.g. buggy kernel code, correct
> kernel code, buggy user space code etc).
>
>        Arnd


Dear Arnd:

I am plans to add the following description in the git commit's comment

Andes processors cannot load/store information which is not naturally aligned
on the bus, i.e., loading a 4 byte data whose start address must be divisible
by 4. If unaligned data accessing is happened, data unaligned exception will be
triggered and user will get SIGSEGV or kernel oops according to the unaligned
address. In order to make user able to load/store data from an unaligned
address, software load /store emulation is implemented in
arch/nds32/mm/alignment.c to handle data unaligned exception.

Unaligned accessing handling is disable by default because of it is not
a normal case. User can follow the steps below to enable this feature.

A. Compile time:
    1. Enable kernel config CONFIG_ALIGNMENT_TRAP
B. Run time:
    1. Enter /proc/sys/nds32/unaligned_acess folder
    2.
        a. Write â1â to file enable_mode to enable unaligned accessing handling
            for user space
        b. Write â2â to file enable_mode to enable unaligned accessing handling
            for kernel space
        c. Write â3â to file enable_mode to enable unaligned accessing handling
            for user and kernel space
        d. Write '0' to file enable_mode to disable all unaligned
accessing handling
    3.
        a. Write â1â to file debug to show which unaligned address is addressed
        b. Write â0â to file debug to disable all information

However, unaligned accessing handler canât work on following conditions.
A. The corresponding mode for unaligned address is not enabled.
B. This unaligned address is not accessible such as protection violation.
On these conditions, the default behaviors for data unaligned
exception still happen.



Thanks

Vincent