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Date:   Thu, 7 Jul 2022 12:22:52 +0800
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Subject: Re: [PATCH v2] LoongArch: Clean useless vcsr in loongarch_fpu.
To:     Xi Ruoyao <xry111@xry111.site>, WANG Xuerui <kernel@xen0n.name>,
        Qi Hu <huqi@loongson.cn>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        Huacai Chen <chenhuacai@kernel.org>
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From:   WANG Xuerui <kernel@xen0n.name>
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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.