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Date: Mon, 17 Oct 2022 16:13:47 +0800 From: Xi Ruoyao <xry111@...111.site> To: Arnd Bergmann <arnd@...db.de>, Huacai Chen <chenhuacai@...nel.org> Cc: Huacai Chen <chenhuacai@...ngson.cn>, loongarch@...ts.linux.dev, Xuefeng Li <lixuefeng@...ngson.cn>, Tiezhu Yang <yangtiezhu@...ngson.cn>, guoren <guoren@...nel.org>, WANG Xuerui <kernel@...0n.name>, Jiaxun Yang <jiaxun.yang@...goat.com>, linux-kernel@...r.kernel.org Subject: Re: [PATCH] LoongArch: Add unaligned access support 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@...111.site> School of Aerospace Science and Technology, Xidian University
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