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From: Xi Ruoyao <xry111@xry111.site>
To: Arnd Bergmann <arnd@arndb.de>, Huacai Chen <chenhuacai@kernel.org>
Cc: Huacai Chen <chenhuacai@loongson.cn>,
	loongarch@lists.linux.dev, Xuefeng Li <lixuefeng@loongson.cn>,
	Tiezhu Yang <yangtiezhu@loongson.cn>, guoren <guoren@kernel.org>,
	WANG Xuerui <kernel@xen0n.name>,
	Jiaxun Yang <jiaxun.yang@flygoat.com>,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH] LoongArch: Add unaligned access support
Date: Mon, 17 Oct 2022 16:13:47 +0800	[thread overview]
Message-ID: <514ccb3b1a2fbd37f238f169eef0f40c89d3e3dd.camel@xry111.site> (raw)
In-Reply-To: <b97afda4-143c-46d2-a6af-dd6a700472ec@app.fastmail.com>

On Mon, 2022-10-17 at 09:38 +0200, Arnd Bergmann wrote:

> > Some unaligned accesses are observed from the kernel network stack, it
> > seems related to whether the packet aligns to IP header or MAC header.
> 
> This is usually a bug in the device driver. It's a fairly common bug
> since the network driver has to ensure the alignment is correct, but
> it's usually fixable, and fixing it results in better performance on
> machines that support unaligned access as well.

Or, maybe a GCC bug is causing -mstrict-align not implemented correctly.

> > And, gcc has a -mstrict-align parameter, if without this, there are
> > unaligned instructions.
> 
> Does this default to strict or non-strict mode? Usually gcc does not
> allow to turn this off on architectures that have no hardware support
> for unaligned access.

On LoongArch the unaligned access support is optional.  An
implementation is allowed to implement it or not.  The software can
determine if it's supported by a CPUCFG instruction.

I think -march=la264 will turn off strict align, but it's not added into
GCC yet.

The GCC default is -mno-strict-align.  I expressed my concern about this
decision when I reviewed the GCC port, but at last they just kept the
decision.  But the kernel already sets -mstrict-align in CFLAGS anyway.

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

      parent reply	other threads:[~2022-10-17  8:14 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2022-10-16 13:34 [PATCH] LoongArch: Add unaligned access support Huacai Chen
2022-10-16 15:04 ` Xi Ruoyao
2022-10-17  0:16   ` Huacai Chen
2022-10-17  5:49     ` WANG Xuerui
2022-10-17  7:12 ` Arnd Bergmann
2022-10-17  7:31   ` Huacai Chen
2022-10-17  7:38     ` Arnd Bergmann
2022-10-17  8:05       ` WANG Xuerui
2022-10-17  8:22         ` Arnd Bergmann
2022-10-17  8:13       ` Xi Ruoyao [this message]

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