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

[Qi Hu]

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.