linux-kernel - Re: [PATCH] LoongArch: Fix and simplify fcsr initialization on execve

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Date: Tue, 02 Jan 2024 16:09:00 +0800
From: Xi Ruoyao <xry111@...111.site>
To: Huacai Chen <chenhuacai@...nel.org>
Cc: WANG Xuerui <kernel@...0n.name>, Eric Biederman <ebiederm@...ssion.com>,
  Kees Cook <keescook@...omium.org>, Tiezhu Yang <yangtiezhu@...ngson.cn>,
 Jinyang He <hejinyang@...ngson.cn>, Jiaxun Yang <jiaxun.yang@...goat.com>, 
 loongarch@...ts.linux.dev, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org,  stable@...r.kernel.org
Subject: Re: [PATCH] LoongArch: Fix and simplify fcsr initialization on
 execve

On Tue, 2024-01-02 at 10:35 +0800, Huacai Chen wrote:

/* snip */

> > The only other architecture setting FCSR in SET_PERSONALITY2 is MIPS.
> > They do this for supporting different FP flavors (NaN encodings etc).
> > which do not exist on LoongArch.  I'm not sure how MIPS evades the issue
> > (or maybe it's just buggy too) as I don't have a running MIPS hardware
> > now.
> I think you can use QEMU. :)

I'll investigate it later.

> > So for LoongArch, just remove the current->thread.fpu.fcsr setting from
> > SET_PERSONALITY2 and do it in start_thread, after lose_fpu(0).  And we
> > just set it to 0, instead of boot_cpu_data.fpu_csr0 (because we should
> > provide the userspace a consistent configuration, no matter how hardware
> > and firmware behave).
> I still prefer to set fcsr to boot_cpu_data.fpu_csr0, because we will
> add LoongArch32 later, not sure whether something will change.

I just seen fpu_csr0 is initialized to FPU_CSR_RN which is just 0 for
LA64, so my concern about firmware & hardware leaving non-zero FCSR is
not valid.  I'll send v2 to keep using boot_cpu_data.fpu_csr0 then.

> 
> 
-- 
Xi Ruoyao <xry111@...111.site>
School of Aerospace Science and Technology, Xidian University