Messages in this thread | | | Date | Thu, 7 Jul 2022 12:22:52 +0800 | Subject | Re: [PATCH v2] LoongArch: Clean useless vcsr in loongarch_fpu. | From | WANG Xuerui <> |
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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.
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