Messages in this thread | | | Subject | Re: [PATCH v2] LoongArch: Clean useless vcsr in loongarch_fpu. | From | Xi Ruoyao <> | Date | Thu, 07 Jul 2022 12:04:28 +0800 |
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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. -- Xi Ruoyao <xry111@xry111.site> School of Aerospace Science and Technology, Xidian University
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