Re: [PATCH v2] LoongArch: Clean useless vcsr in loongarch_fpu.

[Qi Hu <huqi@loongson.cn>]

On 2022/7/7 12:22, WANG Xuerui wrote:
> On 2022/7/7 12:04, Xi Ruoyao wrote:
>> On Thu, 2022-07-07 at 11:05 +0800, WANG Xuerui wrote:
>>
>>> To be frank, at this point I think you're trying to hide something.
>>> (This is not your fault, blame someone else of course because they told
>>> you the fact.) In the old-world kernel the VCSR a.k.a. FCSR16 is
>>> certainly being saved/restored, and there's apparently no harm in doing
>>> so. And if the contents are indeed "undefined", why are the code there
>>> in the first place? Certainly the bits *are* meaningful, only that for
>>> some reason you aren't revealing the semantics and pretending that they
>>> are "undefined" and probably "do nothing externally observable" if
>>> accessed in the first place.
>> On a 3A5000LL, I did an experiment via a kernel module, which enables
>> LSX/LASX and tries to write and read fcsr16.  I tried each bit (1, 2, 4,
>> 8, ..., 1 << 31) one by one.  The result: no matter which bit I wrote
>> into fcsr16, I always read out 0.
>>
>> And I've objdump'ed a kernel shipped in an early Loongnix release.  It
>> seems the only reference to fcsr16 is a "movgr2fcsr $r16, $r0"
>> instruction.
>
> Hmm this is weird. I can't understand why the vcsr code was there in 
> the first place then... I'd like to check a few Loongnix/Kylin/UOS 
> kernels but currently I don't have the time.
>
> If this is the case, indeed all vcsr-related code should be removed. 
> Although I'm still not sure how to best word the commit message.
>
Thanks for the Ruoyao's experiment.

Removing the vcsr is the first step to trying to support LBT and 
LSX/LASX in Kernel. In my opinion, the vcsr relevant code may be used 
for debugging and forgot to remove.