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Date:   Wed, 25 Oct 2023 14:11:45 -0700
From:   Charlie Jenkins <charlie@rivosinc.com>
To:     Arnd Bergmann <arnd@arndb.de>
Cc:     "Wang, Xiao W" <xiao.w.wang@intel.com>,
        Linux-Arch <linux-arch@vger.kernel.org>,
        Albert Ou <aou@eecs.berkeley.edu>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Conor Dooley <conor@kernel.org>,
        David Laight <David.Laight@aculab.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        "linux-riscv@lists.infradead.org" <linux-riscv@lists.infradead.org>
Subject: Re: [PATCH v7 2/4] riscv: Checksum header
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On Wed, Oct 25, 2023 at 10:52:22PM +0200, Arnd Bergmann wrote:
> On Wed, Oct 25, 2023, at 22:37, Charlie Jenkins wrote:
> > On Wed, Oct 25, 2023 at 06:50:05AM +0000, Wang, Xiao W wrote:
> 
> >> > +
> >> > +/*
> >> > + * Quickly compute an IP checksum with the assumption that IPv4 headers
> >> > will
> >> > + * always be in multiples of 32-bits, and have an ihl of at least 5.
> >> > + * @ihl is the number of 32 bit segments and must be greater than or equal
> >> > to 5.
> >> > + * @iph is assumed to be word aligned.
> >> 
> >> Not sure if the assumption is always true. It looks the implementation in "lib/checksum.c" doesn't take this assumption.
> >> The ip header can comes after a 14-Byte ether header, which may start from a word-aligned or DMA friendly address.
> >
> > While lib/checksum.c does not make this assumption, other architectures
> > (x86, ARM, powerpc, mips, arc) do make this assumption. Architectures
> > seem to only align the header on a word boundary in do_csum. I worry
> > that the benefit of aligning iph in this "fast" csum function would
> > disproportionately impact hardware that has fast misaligned accesses.
> 
> Most architectures set NET_IP_ALIGN to '2', which is intended
> to have the IP header at a 32-bit aligned address, though
> some other targets don't bother:
> 
> arch/arm64/include/asm/processor.h:#define NET_IP_ALIGN 0
> arch/powerpc/include/asm/processor.h:#define NET_IP_ALIGN       0
> arch/x86/include/asm/processor.h:#define NET_IP_ALIGN   0
> include/linux/skbuff.h:#define NET_IP_ALIGN     2
> 
> I think it's considered a driver bug if an SKB ends up
> with a misaligned IP header, but it's also something that
> some of the more obscure drivers get wrong.
> 
>     Arnd

Thank you for pointing that out, I had not realized that macro existed.
Since riscv keeps NET_IP_ALIGN at 0 it should be expected that
ip_fast_csum is only called with 32-bit aligned addresses. I will update
the comment and refer to that macro. riscv supports misaligned accesses
but there are no guarantees of speed.

- Charlie