Re: [PATCH v7 2/4] riscv: Checksum header

[Charlie Jenkins <charlie@rivosinc.com>]

On Wed, Oct 25, 2023 at 06:50:05AM +0000, Wang, Xiao W wrote:
> Hi Charlie,
> 
> > -----Original Message-----
> > From: linux-riscv <linux-riscv-bounces@lists.infradead.org> On Behalf Of
> > Charlie Jenkins
> > Sent: Wednesday, September 20, 2023 2:45 AM
> > To: Charlie Jenkins <charlie@rivosinc.com>; Palmer Dabbelt
> > <palmer@dabbelt.com>; Conor Dooley <conor@kernel.org>; Samuel Holland
> > <samuel.holland@sifive.com>; David Laight <David.Laight@aculab.com>;
> > linux-riscv@lists.infradead.org; linux-kernel@vger.kernel.org; linux-
> > arch@vger.kernel.org
> > Cc: Paul Walmsley <paul.walmsley@sifive.com>; Albert Ou
> > <aou@eecs.berkeley.edu>; Arnd Bergmann <arnd@arndb.de>
> > Subject: [PATCH v7 2/4] riscv: Checksum header
> > 
> > Provide checksum algorithms that have been designed to leverage riscv
> > instructions such as rotate. In 64-bit, can take advantage of the larger
> > register to avoid some overflow checking.
> > 
> > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > ---
> >  arch/riscv/include/asm/checksum.h | 79
> > +++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 79 insertions(+)
> > 
> > diff --git a/arch/riscv/include/asm/checksum.h
> > b/arch/riscv/include/asm/checksum.h
> > new file mode 100644
> > index 000000000000..dc0dd89f2a13
> > --- /dev/null
> > +++ b/arch/riscv/include/asm/checksum.h
> > @@ -0,0 +1,79 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * IP checksum routines
> > + *
> > + * Copyright (C) 2023 Rivos Inc.
> > + */
> > +#ifndef __ASM_RISCV_CHECKSUM_H
> > +#define __ASM_RISCV_CHECKSUM_H
> > +
> > +#include <linux/in6.h>
> > +#include <linux/uaccess.h>
> > +
> > +#define ip_fast_csum ip_fast_csum
> > +
> > +#include <asm-generic/checksum.h>
> > +
> > +/*
> > + * 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.

- Charlie

> 
> > + */
> > +static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
> > +{
> > +	unsigned long csum = 0;
> > +	int pos = 0;
> > +
> > +	do {
> > +		csum += ((const unsigned int *)iph)[pos];
> > +		if (IS_ENABLED(CONFIG_32BIT))
> > +			csum += csum < ((const unsigned int *)iph)[pos];
> > +	} while (++pos < ihl);
> > +
> > +	/*
> > +	 * ZBB only saves three instructions on 32-bit and five on 64-bit so not
> > +	 * worth checking if supported without Alternatives.
> > +	 */
> > +	if (IS_ENABLED(CONFIG_RISCV_ISA_ZBB) &&
> > +	    IS_ENABLED(CONFIG_RISCV_ALTERNATIVE)) {
> > +		unsigned long fold_temp;
> > +
> > +		asm_volatile_goto(ALTERNATIVE("j %l[no_zbb]", "nop", 0,
> > +					      RISCV_ISA_EXT_ZBB, 1)
> > +		    :
> > +		    :
> > +		    :
> > +		    : no_zbb);
> > +
> > +		if (IS_ENABLED(CONFIG_32BIT)) {
> > +			asm(".option push				\n\
> > +			.option arch,+zbb				\n\
> > +				not	%[fold_temp], %[csum]
> > 	\n\
> > +				rori	%[csum], %[csum], 16		\n\
> > +				sub	%[csum], %[fold_temp], %[csum]
> > 	\n\
> > +			.option pop"
> > +			: [csum] "+r" (csum), [fold_temp] "=&r" (fold_temp));
> > +		} else {
> > +			asm(".option push				\n\
> > +			.option arch,+zbb				\n\
> > +				rori	%[fold_temp], %[csum], 32	\n\
> > +				add	%[csum], %[fold_temp], %[csum]
> > 	\n\
> > +				srli	%[csum], %[csum], 32		\n\
> > +				not	%[fold_temp], %[csum]
> > 	\n\
> > +				roriw	%[csum], %[csum], 16		\n\
> > +				subw	%[csum], %[fold_temp], %[csum]
> > 	\n\
> > +			.option pop"
> > +			: [csum] "+r" (csum), [fold_temp] "=&r" (fold_temp));
> > +		}
> > +		return csum >> 16;
> > +	}
> > +no_zbb:
> > +#ifndef CONFIG_32BIT
> > +	csum += (csum >> 32) | (csum << 32);
> 
> Just like patch 3/4 does, we can call ror64(csum, 32).
> 
> BRs,
> Xiao
> 
> > +	csum >>= 32;
> > +#endif
> > +	return csum_fold((__force __wsum)csum);
> > +}
> > +
> > +#endif // __ASM_RISCV_CHECKSUM_H
> > 
> > --
> > 2.42.0
> > 
> > 
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