linux-kernel - Re: [PATCH v2] LoongArch: Clean useless vcsr in loongarch

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Date:   Thu, 7 Jul 2022 14:39:35 +0800
From:   Qi Hu <huqi@...ngson.cn>
To:     WANG Xuerui <kernel@...0n.name>, Xi Ruoyao <xry111@...111.site>,
        Jiaxun Yang <jiaxun.yang@...goat.com>,
        Huacai Chen <chenhuacai@...nel.org>
Cc:     loongarch@...ts.linux.dev, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] LoongArch: Clean useless vcsr in loongarch_fpu.


On 2022/7/7 14:24, WANG Xuerui wrote:
>
> On 2022/7/7 14:09, Qi Hu wrote:
>>
>> 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.
>>
> Thinking about it harder, it actually makes sense. Given that access 
> to the LSX/LASX manuals is currently restricted, outsiders can never 
> know whether the code in question is really needed, so one has to err 
> on the conservative side. Thanks for the clarification, and my 
> apologies for being harsh in the previous reply.
>
> I think the commit message could be reworded like:
>
> "The VCSR (also known as FCSR16) is not present on retail steppings of 
> 3A5000. FCSR16 through FCSR31 are reserved for non-floating-point 
> LSX/LASX operations, but on 3A5000 all writes to them are no-op and 
> all reads return zero. FP LSX/LASX operations reuse the FCSR0 as their 
> CSR.
>
> Remove VCSR handling that is probably leftover debugging code for an 
> earlier not-for-retail stepping."
>
> (And remove the trailing period in your patch title, while at it; the 
> Linux kernel doesn't use a trailing period for majority of its commits.)
>
Thanks for reminding me about this. It is my first time committing the 
patch to Linux Kernel, and the commit message will be modified at V4 patch.

-

Qi