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    SubjectRe: [PATCH] arm64: Implement optimised IP checksum helpers
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    Hi Robin,

    I pulled this in to a userspace test app expecting that the __uint128_t
    type might cause GCC to emit 'ldp'. Seems like that was that your
    intent based on your commit note. Instead I see two 64b loads (ldr Xn),
    and a single 32b load (ldr Wn) for the trailing 4B. This was with
    Linaro GCC 4.9-2015.06.

    Otherwise, the C cycle count looks good enough compared to the asm version.

    Thanks,

    Luke


    On 5/12/2016 10:26 AM, Robin Murphy wrote:
    > AArch64 is capable of 128-bit memory accesses without alignment
    > restrictions, which makes it both possible and highly practical to slurp
    > up a typical 20-byte IP header in just 2 loads. Implement our own
    > version of ip_fast_checksum() to take advantage of that, resulting in
    > considerably fewer instructions and memory accesses than the generic
    > version. We can also get more optimal code generation for csum_fold() by
    > defining it a slightly different way round from the generic version, so
    > throw that into the mix too.
    >
    > Suggested-by: Luke Starrett <luke.starrett@broadcom.com>
    > Signed-off-by: Robin Murphy <robin.murphy@arm.com>
    > ---
    >
    > Having spent an evening poring over disassembler output, it turns out
    > that there's really no need to drop into asm for this, hurrah! I've
    > taken it for a spin on both big and little-endian, with no ill effects.
    >
    > arch/arm64/include/asm/checksum.h | 49 +++++++++++++++++++++++++++++++++++++++
    > 1 file changed, 49 insertions(+)
    > create mode 100644 arch/arm64/include/asm/checksum.h
    >
    > diff --git a/arch/arm64/include/asm/checksum.h b/arch/arm64/include/asm/checksum.h
    > new file mode 100644
    > index 000000000000..7e975ffce837
    > --- /dev/null
    > +++ b/arch/arm64/include/asm/checksum.h
    > @@ -0,0 +1,49 @@
    > +/*
    > + * Copyright (C) 2016 ARM Ltd.
    > + *
    > + * This program is free software; you can redistribute it and/or modify
    > + * it under the terms of the GNU General Public License version 2 as
    > + * published by the Free Software Foundation.
    > + *
    > + * This program is distributed in the hope that it will be useful,
    > + * but WITHOUT ANY WARRANTY; without even the implied warranty of
    > + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
    > + * GNU General Public License for more details.
    > + *
    > + * You should have received a copy of the GNU General Public License
    > + * along with this program. If not, see <http://www.gnu.org/licenses/>.
    > + */
    > +#ifndef __ASM_CHECKSUM_H
    > +#define __ASM_CHECKSUM_H
    > +
    > +static inline __sum16 csum_fold(__wsum csum)
    > +{
    > + u32 sum = (__force u32)csum;
    > + sum += (sum >> 16) | (sum << 16);
    > + return ~(__force __sum16)(sum >> 16);
    > +}
    > +#define csum_fold csum_fold
    > +
    > +static inline __sum16 ip_fast_csum(const void *iph, unsigned int ihl)
    > +{
    > + __uint128_t tmp;
    > + u64 sum;
    > +
    > + tmp = *(const __uint128_t *)iph;
    > + iph += 16;
    > + ihl -= 4;
    > + tmp += ((tmp >> 64) | (tmp << 64));
    > + sum = tmp >> 64;
    > + do {
    > + sum += *(const u32 *)iph;
    > + iph += 4;
    > + } while (--ihl);
    > +
    > + sum += ((sum >> 32) | (sum << 32));
    > + return csum_fold(sum >> 32);
    > +}
    > +#define ip_fast_csum ip_fast_csum
    > +
    > +#include <asm-generic/checksum.h>
    > +
    > +#endif /* __ASM_CHECKSUM_H */

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