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

[WANG Xuerui]

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.