Re: checksum: add VSX fast path for POWER8/POWER9

[jfiusdq <jfiusdq@proton.me>]

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Microbenchmark of the checksum function vs C version at different buffer sizes:

Results (GB/s, higher is better; speedup = VSX / scalar):

64B: VSX 4.61 vs scalar 5.91 -> 0.78x (VSX slower for tiny buffers)
256B: VSX 10.91 vs scalar 7.57 -> 1.44x
1500B: VSX 13.88 vs scalar 6.89 -> 2.02x
16KB: VSX 14.53 vs scalar 6.96 -> 2.09x
64KB: VSX 15.15 vs scalar 6.85 -> 2.21x

On Friday, February 6th, 2026 at 3:17 PM, Laurent Vivier <lvivier@redhat.com> wrote:

> 

> 

> On Thu, 05 Feb 2026 06:14:40 +0000, jfiusdq jfiusdq@proton.me wrote:
> 

> > Tested with podman on Debian 13 for a while and works ok. It's
> > difficult to run all the tests on POWER but 505-networking-pasta.bats
> > test suite passes.
> > ---
> > checksum.c | 110 +++++++++++++++++++++++++++++++++++++++++++++++++++--
> > 1 file changed, 107 insertions(+), 3 deletions(-)
> > 

> > diff --git a/checksum.c b/checksum.c
> > index 0c3837c..828f9ec 100644
> > --- a/checksum.c
> > +++ b/checksum.c
> > @@ -281,7 +281,7 @@ void csum_icmp6(struct icmp6hdr *icmp6hr,
> > icmp6hr->icmp6_cksum = csum(payload, dlen, psum);
> > }
> > 

> > -#ifdef AVX2
> > +#if defined(AVX2)
> > #include <immintrin.h>
> > 

> > /**
> > @@ -479,7 +479,111 @@ uint32_t csum_unfolded(const void *buf, size_t len, uint32_t init)
> > 

> > return init;
> > }
> > -#else /* AVX2 /
> > +#elif defined(POWER9_VECTOR) || defined(POWER8_VECTOR)
> > +#include <altivec.h>
> > +
> > +/*
> > + * csum_vsx() - Compute 32-bit checksum using VSX SIMD instructions
> > + * @buf: Input buffer
> > + * @len: Input length
> > + * @init: Initial 32-bit checksum, 0 for no pre-computed checksum
> > + *
> > + * Return: 32-bit checksum, not complemented, not folded
> > + /
> > +/ NOLINTNEXTLINE(clang-diagnostic-unknown-attributes) /
> > +attribute((optimize("-fno-strict-aliasing"))) / See csum_16b() */
> > +static uint32_t csum_vsx(const void buf, size_t len, uint32_t init)
> > +{
> > + const uint8_t p = buf;
> > + vector unsigned int sum_even = vec_splat_u32(0);
> > + vector unsigned int sum_odd = vec_splat_u32(0);
> > + const vector unsigned short ones = vec_splat_u16(1);
> > + uint64_t sum64 = init;
> > +
> > +#ifdef POWER9_VECTOR
> > + while (len >= 64) {
> > + vector unsigned char v0b = vec_vsx_ld(0, p);
> > + vector unsigned char v1b = vec_vsx_ld(16, p);
> > + vector unsigned char v2b = vec_vsx_ld(32, p);
> > + vector unsigned char v3b = vec_vsx_ld(48, p);
> > + vector unsigned short v0 = (vector unsigned short)v0b;
> > + vector unsigned short v1 = (vector unsigned short)v1b;
> > + vector unsigned short v2 = (vector unsigned short)v2b;
> > + vector unsigned short v3 = (vector unsigned short)v3b;
> > +
> > + sum_even = vec_add(sum_even, vec_mule(v0, ones));
> > + sum_odd = vec_add(sum_odd, vec_mulo(v0, ones));
> > + sum_even = vec_add(sum_even, vec_mule(v1, ones));
> > + sum_odd = vec_add(sum_odd, vec_mulo(v1, ones));
> > + sum_even = vec_add(sum_even, vec_mule(v2, ones));
> > + sum_odd = vec_add(sum_odd, vec_mulo(v2, ones));
> > + sum_even = vec_add(sum_even, vec_mule(v3, ones));
> > + sum_odd = vec_add(sum_odd, vec_mulo(v3, ones));
> > +
> > + p += 64;
> > + len -= 64;
> > + }
> > +#endif
> > +
> > + while (len >= 32) {
> > + vector unsigned char v0b = vec_vsx_ld(0, p);
> > + vector unsigned char v1b = vec_vsx_ld(16, p);
> > + vector unsigned short v0 = (vector unsigned short)v0b;
> > + vector unsigned short v1 = (vector unsigned short)v1b;
> > +
> > + sum_even = vec_add(sum_even, vec_mule(v0, ones));
> > + sum_odd = vec_add(sum_odd, vec_mulo(v0, ones));
> > + sum_even = vec_add(sum_even, vec_mule(v1, ones));
> > + sum_odd = vec_add(sum_odd, vec_mulo(v1, ones));
> > +
> > + p += 32;
> > + len -= 32;
> > + }
> > +
> > + while (len >= 16) {
> > + vector unsigned char v0b = vec_vsx_ld(0, p);
> > + vector unsigned short v0 = (vector unsigned short)v0b;
> > +
> > + sum_even = vec_add(sum_even, vec_mule(v0, ones));
> > + sum_odd = vec_add(sum_odd, vec_mulo(v0, ones));
> > +
> > + p += 16;
> > + len -= 16;
> > + }
> > +
> > + {
> > + vector unsigned int sum32 = vec_add(sum_even, sum_odd);
> > + uint32_t partial[4] attribute((aligned(16)));
> > +
> > + vec_st(sum32, 0, partial);
> > + sum64 += (uint64_t)partial[0] + partial[1] +
> > + partial[2] + partial[3];
> > + }
> > +
> > + sum64 += sum_16b(p, len);
> > +
> > + sum64 = (sum64 >> 32) + (sum64 & 0xffffffff);
> > + sum64 += sum64 >> 32;
> > +
> > + return (uint32_t)sum64;
> > +}
> > +
> > +/
> > + * csum_unfolded() - Calculate the unfolded checksum of a data buffer.
> > + *
> > + * @buf: Input buffer
> > + * @len: Input length
> > + * @init: Initial 32-bit checksum, 0 for no pre-computed checksum
> > + *
> > + * Return: 32-bit unfolded checksum
> > + /
> > +/ NOLINTNEXTLINE(clang-diagnostic-unknown-attributes) /
> > +attribute((optimize("-fno-strict-aliasing"))) / See csum_16b() /
> > +uint32_t csum_unfolded(const void buf, size_t len, uint32_t init)
> > +{
> > + return csum_vsx(buf, len, init);
> > +}
> > +#else / !AVX2 && !POWER9_VECTOR && !POWER8_VECTOR /
> > /
> > * csum_unfolded() - Calculate the unfolded checksum of a data buffer.
> > *
> > @@ -495,7 +599,7 @@ uint32_t csum_unfolded(const void buf, size_t len, uint32_t init)
> > {
> > return sum_16b(buf, len) + init;
> > }
> > -#endif / !AVX2 /
> > +#endif / !AVX2 && !POWER9_VECTOR && !POWER8_VECTOR */
> > 

> > /**
> > * csum_iov_tail() - Calculate unfolded checksum for the tail of an IO vector
> > --
> > 2.52.0
> 

> 

> Reviewed-by: Laurent Vivier lvivier@redhat.com

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