Re: [PATCH v9 3/5] riscv: Checksum header

[Charlie Jenkins]

On Wed, Nov 01, 2023 at 11:22:52PM +0800, Jisheng Zhang wrote:
> On Tue, Oct 31, 2023 at 05:18:53PM -0700, Charlie Jenkins wrote:
> > 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@xxxxxxxxxxxx>
> > Acked-by: Conor Dooley <conor.dooley@xxxxxxxxxxxxx>
> > ---
> >  arch/riscv/include/asm/checksum.h | 81 +++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 81 insertions(+)
> > 
> > diff --git a/arch/riscv/include/asm/checksum.h b/arch/riscv/include/asm/checksum.h
> > new file mode 100644
> > index 000000000000..3d77cac338fe
> > --- /dev/null
> > +++ b/arch/riscv/include/asm/checksum.h
> > @@ -0,0 +1,81 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * 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
> > +
> > +/* Define riscv versions of functions before importing asm-generic/checksum.h */
> > +#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 given that NET_IP_ALIGN is set to 2 on
> > + *	riscv, defining IP headers to be aligned.
> > + */
> > +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)
> 
> This looks like a open coding of riscv_has_extension_*, so if
> we use the it, the code could be rewritten as:
> 
> 	if (riscv_has_extension_likely(RISCV_ISA_EXT_ZBB)) {
> 		if (32bit) {
> 			asm(...)
> 
> 		} else {
> 			asm(...)
> 		}
> 		return csum >> 16;
> 	}
> #ifndef CONFIG_32BIT
> 	csum += ror64(csum, 32);
> 	csum >>= 32;
> #endif
> 	return csum_fold((__force __wsum)csum);
> 
> The code readability is improved and make it a bit easier to refactor
> the asm(...) code in the future.
> 
> And IMHO, the generated code should be the same.
> 
> Thanks

It is unfortunately not the same. riscv_has_extension_likely checks at
runtime for ZBB if alternatives is not supported. That is why the
comment is there. I can reference riscv_has_extension_likely to avoid
confusion.

- Charlie

> 
> >
> 
> > +		    :
> > +		    :
> > +		    :
> > +		    : 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 += ror64(csum, 32);
> > +	csum >>= 32;
> > +#endif
> > +	return csum_fold((__force __wsum)csum);
> > +}
> > +
> > +#endif /* __ASM_RISCV_CHECKSUM_H */
> > 
> > -- 
> > 2.34.1
> > 
> > 
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