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

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Message-ID: <22a1ba993e298ce12a374decefebeca484240883.camel@xry111.site>
Date:   Tue, 05 Jul 2022 17:49:22 +0800
From:   Xi Ruoyao <xry111@...111.site>
To:     WANG Xuerui <kernel@...0n.name>, Qi Hu <huqi@...ngson.cn>,
        Huacai Chen <chenhuacai@...nel.org>,
        Jiaxun Yang <jiaxun.yang@...goat.com>
Cc:     loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] LoongArch: Clean useless vcsr in loongarch_fpu.

On Tue, 2022-07-05 at 16:46 +0800, WANG Xuerui wrote:

> Actually I'm still not very satisfied with the explanation; the code 
> must be written with *something* in mind, since GS464V/LA464 is the only 
> LoongArch implementation so far, it must have a VCSR to begin with. And 
> you can't magically melt the VCSR on the tens of thousands of 
> 3A5000/3C5000's already shipped, because the old-world kernel obviously 
> comes with LSX/LASX and it most likely utilizes the VCSR. In addition,
> you didn't mention what will happen if LSX/LASX *is* enabled on this 
> new-world kernel, *and* fcsr16 is being accessed.
> 
> I think maybe you just want to remove the mentions of VCSR since they 
> are dead code right now, as I don't believe it's gone in shipped 
> hardware, as explained above. Except if there's magically a way to 
> implement LSX/LASX without touching the said-to-have-disappeared VCSR,
> which I don't know of, and can't know because the LSX/LASX ISA manual is 
> still not publicly accessible; so I don't feel comfortable approving 
> this patch.

Let me make some bold guess here.  In the MIPS-compatible 3A4000 we had
"MSACSR" register.  According to the MSA manual, only the 24-th bit of
this register is used:

"Some MSA floating point instructions might not handle subnormal input
operands or compute tiny non-zero results. Such instructions may signal
the Unimplemented Operation Exception and let the software emulation
finalize the operation. If software emulation is not needed or desired,
MSACSR FS bit could be set to replace every tiny non-zero result and
subnormal input operand with zero of the same sign."

And, it says:

"Should the alternate exception handling attributes of the IEEE Standard
for Floating-Point Arithmetic 754-2008, Section 8 be desired, the MSACSR
FS bit should be zero, the Underflow Exception be enabled and a trap
handler be provided to carry out the execution of the alternate
exception handling attributes."

We can see Loongson has been trying to make 3A processors more IEEE-754
compatible.  For example, 3A4000 is the only non-R6 MIPS-compatible
processor using IEEE-754-2008 NaN encoding.  And LoongArch manual
directly refers to IEEE-754-2008 manual in many places.  So I guess this
change means Loongson won't set this bit to 1 for 3A5000 at all, and any
attempt by the user to set this bit could be considered undefined
behavior (causing inconsistent behavior on 3A5000 as the kernel doesn't
save/restore VCSR at context switch, and SIGILL on 3A6000 as the
register is removed).
-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University