Re: [PATCH v4 2/5] riscv: Add checksum library

From: Charlie Jenkins
Date: Thu Sep 14 2023 - 13:58:47 EST

On Thu, Sep 14, 2023 at 01:25:23PM +0100, Conor Dooley wrote:
> On Mon, Sep 11, 2023 at 03:57:12PM -0700, Charlie Jenkins wrote:
> > Provide a 32 and 64 bit version of do_csum. When compiled for 32-bit
> > will load from the buffer in groups of 32 bits, and when compiled for
> > 64-bit will load in groups of 64 bits. Benchmarking by proxy compiling
> > csum_ipv6_magic (64-bit version) for an x86 chip as well as running
> > the riscv generated code in QEMU, discovered that summing in a
> > tree-like structure is about 4% faster than doing 64-bit reads.
> >
> > Signed-off-by: Charlie Jenkins <charlie@xxxxxxxxxxxx>
> > ---
> > arch/riscv/include/asm/checksum.h | 11 ++
> > arch/riscv/lib/Makefile | 1 +
> > arch/riscv/lib/csum.c | 210 ++++++++++++++++++++++++++++++++++++++
> > 3 files changed, 222 insertions(+)
> >
> > diff --git a/arch/riscv/include/asm/checksum.h b/arch/riscv/include/asm/checksum.h
> > index 0d7fc8275a5e..a09a4053fb87 100644
> > --- a/arch/riscv/include/asm/checksum.h
> > +++ b/arch/riscv/include/asm/checksum.h
> > @@ -16,6 +16,14 @@ typedef unsigned int csum_t;
> > typedef unsigned long csum_t;
> > #endif
> >
> > +/* Default version is sufficient for 32 bit */
> > +#ifdef CONFIG_64BIT
> > +#define _HAVE_ARCH_IPV6_CSUM
> > +__sum16 csum_ipv6_magic(const struct in6_addr *saddr,
> > + const struct in6_addr *daddr,
> > + __u32 len, __u8 proto, __wsum sum);
> > +#endif
> > +
> > /*
> > * Fold a partial checksum without adding pseudo headers
> > */
> > @@ -90,6 +98,9 @@ static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
> >
> > #define ip_fast_csum ip_fast_csum
> >
> > +extern unsigned int do_csum(const unsigned char *buff, int len);
> > +#define do_csum do_csum
> > +
> > #include <asm-generic/checksum.h>
> >
> > #endif // __ASM_RISCV_CHECKSUM_H
> > diff --git a/arch/riscv/lib/Makefile b/arch/riscv/lib/Makefile
> > index 26cb2502ecf8..2aa1a4ad361f 100644
> > --- a/arch/riscv/lib/Makefile
> > +++ b/arch/riscv/lib/Makefile
> > @@ -6,6 +6,7 @@ lib-y += memmove.o
> > lib-y += strcmp.o
> > lib-y += strlen.o
> > lib-y += strncmp.o
> > +lib-y += csum.o
> > lib-$(CONFIG_MMU) += uaccess.o
> > lib-$(CONFIG_64BIT) += tishift.o
> > lib-$(CONFIG_RISCV_ISA_ZICBOZ) += clear_page.o
> > diff --git a/arch/riscv/lib/csum.c b/arch/riscv/lib/csum.c
> > new file mode 100644
> > index 000000000000..47d98c51bab2
> > --- /dev/null
> > +++ b/arch/riscv/lib/csum.c
> > @@ -0,0 +1,210 @@
> > +// SPDX-License-Identifier: GPL-2.0
> > +/*
> > + * IP checksum library
> > + *
> > + * Influenced by arch/arm64/lib/csum.c
> > + * Copyright (C) 2023 Rivos Inc.
> > + */
> > +#include <linux/bitops.h>
> > +#include <linux/compiler.h>
> > +#include <linux/kasan-checks.h>
> > +#include <linux/kernel.h>
> > +
> > +#include <net/checksum.h>
> > +
> > +/* Default version is sufficient for 32 bit */
> > +#ifndef CONFIG_32BIT
> > +__sum16 csum_ipv6_magic(const struct in6_addr *saddr,
> > + const struct in6_addr *daddr,
> > + __u32 len, __u8 proto, __wsum csum)
> > +{
> > + /*
> > + * Inform the compiler/processor that the operation we are performing is
> > + * "Commutative and Associative" by summing parts of the checksum in a
> > + * tree-like structure (Section 2(A) of "Computing the Internet
> > + * Checksum"). Furthermore, defer the overflow until the end of the
> > + * computation which is shown to be valid in Section 2(C)(1) of the
> > + * same handbook.
> > + */
> > + unsigned long sum, sum1, sum2, sum3, sum4, ulen, uproto;
> > +
> > + uproto = htonl(proto);
> > + ulen = htonl(len);
> > +
> > + sum = saddr->s6_addr32[0];
> > + sum += saddr->s6_addr32[1];
> > + sum1 = saddr->s6_addr32[2];
> > + sum1 += saddr->s6_addr32[3];
> > +
> > + sum2 = daddr->s6_addr32[0];
> > + sum2 += daddr->s6_addr32[1];
> > + sum3 = daddr->s6_addr32[2];
> > + sum3 += daddr->s6_addr32[3];
> > +
> > + sum4 = csum;
> > + sum4 += ulen;
> > + sum4 += uproto;
> > +
> > + sum += sum1;
> > + sum2 += sum3;
> > +
> > + sum += sum2;
> > + sum += sum4;
> > +
> > + if (IS_ENABLED(CONFIG_RISCV_ISA_ZBB) &&
> > + IS_ENABLED(CONFIG_RISCV_ALTERNATIVE)) {
> > + csum_t fold_temp;
> > +
> > + /*
> > + * Zbb is likely available when the kernel is compiled with Zbb
> > + * support, so nop when Zbb is available and jump when Zbb is
> > + * not available.
> > + */
> > + asm_volatile_goto(ALTERNATIVE("j %l[no_zbb]", "nop", 0,
> > + RISCV_ISA_EXT_ZBB, 1)
> > + :
> > + :
> > + :
> > + : no_zbb);
> > + asm(".option push \n\
> > + .option arch,+zbb \n\
> > + rori %[fold_temp], %[sum], 32 \n\
> > + add %[sum], %[fold_temp], %[sum] \n\
> > + srli %[sum], %[sum], 32 \n\
> > + not %[fold_temp], %[sum] \n\
> > + roriw %[sum], %[sum], 16 \n\
> > + subw %[sum], %[fold_temp], %[sum] \n\
> > + .option pop"
> > + : [sum] "+r" (sum), [fold_temp] "=&r" (fold_temp));
> > + return (__force __sum16)(sum >> 16);
> > + }
> > +no_zbb:
> > + sum += (sum >> 32) | (sum << 32);
> > + sum >>= 32;
> > + return csum_fold((__force __wsum)sum);
> > +}
> > +EXPORT_SYMBOL(csum_ipv6_magic);
> > +#endif // !CONFIG_32BIT
> > +
> > +#ifdef CONFIG_32BIT
> > +#define OFFSET_MASK 3
> > +#elif CONFIG_64BIT
> > +#define OFFSET_MASK 7
> > +#endif
> > +
> > +/*
> > + * Perform a checksum on an arbitrary memory address.
> > + * Algorithm accounts for buff being misaligned.
> > + * If buff is not aligned, will over-read bytes but not use the bytes that it
> > + * shouldn't. The same thing will occur on the tail-end of the read.
> > + */
> > +unsigned int __no_sanitize_address do_csum(const unsigned char *buff, int len)
> > +{
> > + unsigned int offset, shift;
> > + csum_t csum, data;
> > + const csum_t *ptr;
> > +
> > + if (unlikely(len <= 0))
> > + return 0;
> > + /*
> > + * To align the address, grab the whole first byte in buff.
> > + * Since it is inside of a same byte, it will never cross pages or cache
> > + * lines.
> > + * Directly call KASAN with the alignment we will be using.
> > + */
> > + offset = (csum_t)buff & OFFSET_MASK;
> > + kasan_check_read(buff, len);
> > + ptr = (const csum_t *)(buff - offset);
> > + len = len + offset - sizeof(csum_t);
> > +
> > + /*
> > + * Clear the most signifant bits that were over-read if buff was not
> > + * aligned.
> > + */
> > + shift = offset * 8;
> > + data = *ptr;
> > +#ifdef __LITTLE_ENDIAN
> > + data = (data >> shift) << shift;
> > +#else
> > + data = (data << shift) >> shift;
> > +#endif
> > + /*
> > + * Do 32-bit reads on RV32 and 64-bit reads otherwise. This should be
> > + * faster than doing 32-bit reads on architectures that support larger
> > + * reads.
> > + */
> > + while (len > 0) {
> > + csum += data;
>
> arch/riscv/lib/csum.c:137:3: warning: variable 'csum' is uninitialized when used here [-Wuninitialized]
> csum += data;
> ^~~~
> arch/riscv/lib/csum.c:104:13: note: initialize the variable 'csum' to silence this warning
> csum_t csum, data;
> ^
> = 0
> > + csum += csum < data;
> > + len -= sizeof(csum_t);
> > + ptr += 1;
> > + data = *ptr;
> > + }
> > +
> > + /*
> > + * Perform alignment (and over-read) bytes on the tail if any bytes
> > + * leftover.
> > + */
> > + shift = len * -8;
> > +#ifdef __LITTLE_ENDIAN
> > + data = (data << shift) >> shift;
> > +#else
> > + data = (data >> shift) << shift;
> > +#endif
> > + csum += data;
> > + csum += csum < data;
> > +
> > + if (!riscv_has_extension_likely(RISCV_ISA_EXT_ZBB))
> > + goto no_zbb;
>
> I think this is missing a change for IS_ENABLED(CONFIG_RISCV_ISA_ZBB)?
> arch/riscv/lib/csum.c:180:1: warning: unknown option, expected 'push', 'pop', 'rvc', 'norvc', 'relax' or 'norelax' [-Winline-asm]
> .option arch,+zbb \n\
> ^
> <inline asm>:2:11: note: instantiated into assembly here
> .option arch,+zbb
> ^
> arch/riscv/lib/csum.c:181:1: error: instruction requires the following: 'Zbb' (Basic Bit-Manipulation) or 'Zbkb' (Bitmanip instructions for Cryptography)
> rori %[fold_temp], %[csum], 32 \n\
> ^
> <inline asm>:3:4: note: instantiated into assembly here
> rori a2, a0, 32
> ^
> arch/riscv/lib/csum.c:184:1: error: instruction requires the following: 'Zbb' (Basic Bit-Manipulation) or 'Zbkb' (Bitmanip instructions for Cryptography)
> roriw %[fold_temp], %[csum], 16 \n\
> ^
> <inline asm>:6:4: note: instantiated into assembly here
> roriw a2, a0, 16
> ^
> arch/riscv/lib/csum.c:188:1: error: instruction requires the following: 'Zbb' (Basic Bit-Manipulation) or 'Zbkb' (Bitmanip instructions for Cryptography)
> rev8 %[csum], %[csum] \n\
> ^
> <inline asm>:10:4: note: instantiated into assembly here
> rev8 a0, a0
> ^
> 2 warnings and 3 errors generated.
>
> clang before 17 doesn't support `.option arch`, so the guard is required
> around any code using it. You've got the guard on the other `.option
> arch` user above, so that one seems to have escaped notice ;)
>
> Going forward, it'd be good to test this stuff with non-latest clang to
> make sure you appropriately consider the !Zbb cases.
>
Yes this was an oversight to drop the guard.
>
> > +
> > + unsigned int fold_temp;
> > +
> > + if (IS_ENABLED(CONFIG_32BIT)) {
> > + asm_volatile_goto(".option push \n\
> > + .option arch,+zbb \n\
> > + rori %[fold_temp], %[csum], 16 \n\
> > + andi %[offset], %[offset], 1 \n\
> > + add %[csum], %[fold_temp], %[csum] \n\
> > + beq %[offset], zero, %l[end] \n\
> > + rev8 %[csum], %[csum] \n\
> > + zext.h %[csum], %[csum] \n\
> > + .option pop"
> > + : [csum] "+r" (csum), [fold_temp] "=&r" (fold_temp)
> > + : [offset] "r" (offset)
> > + :
> > + : end);
> > +
> > + return csum;
> > + } else {
> > + asm_volatile_goto(".option push \n\
> > + .option arch,+zbb \n\
> > + rori %[fold_temp], %[csum], 32 \n\
> > + add %[csum], %[fold_temp], %[csum] \n\
> > + srli %[csum], %[csum], 32 \n\
> > + roriw %[fold_temp], %[csum], 16 \n\
> > + addw %[csum], %[fold_temp], %[csum] \n\
> > + andi %[offset], %[offset], 1 \n\
> > + beq %[offset], zero, %l[end] \n\
> > + rev8 %[csum], %[csum] \n\
> > + srli %[csum], %[csum], 32 \n\
> > + zext.h %[csum], %[csum] \n\
> > + .option pop"
> > + : [csum] "+r" (csum), [fold_temp] "=&r" (fold_temp)
> > + : [offset] "r" (offset)
> > + :
> > + : end);
> > +
> > + return csum;
> > + }
> > +end:
> > + return csum >> 16;
> > +no_zbb:
> > +#ifndef CONFIG_32BIT
>
> These can also be moved to IS_ENABLED() FYI, since there's no 32-bit
> stuff here that'd break the build for 64-bit. Ditto elsewhere where
> you've got similar stuff.
>
> Cheers,
> Conor.
This is an ifndef, so 32-bit compilation would throw a warning about
shifting by 32 bits if IS_ENABLED was used instead.

- Charlie
>
> > + csum += (csum >> 32) | (csum << 32);
> > + csum >>= 32;
> > +#endif
> > + csum = (unsigned int)csum + (((unsigned int)csum >> 16) | ((unsigned int)csum << 16));
> > + if (offset & 1)
> > + return (unsigned short)swab32(csum);
> > + return csum >> 16;
> > +}
> >
> > --
> > 2.42.0
> >