linux-kernel - Re: [PATCH] arm64: Implement optimised IP checksum helpers

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Date:	Thu, 12 May 2016 18:08:41 +0100
From:	Robin Murphy <robin.murphy@....com>
To:	Luke Starrett <luke.starrett@...adcom.com>, will.deacon@....com,
	catalin.marinas@....com
Cc:	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	bcm-kernel-feedback-list@...adcom.com
Subject: Re: [PATCH] arm64: Implement optimised IP checksum helpers

Hi Luke,

On 12/05/16 16:34, Luke Starrett wrote:
> 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.

GCC 5 happily emits ldp there, but indeed I couldn't figure out how to 
convince GCC 4 to do so. From a quick ferret around in the GCC Git, it 
looks like the relevant optimisations may have only gone in post-4.9.

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

Yeah, compiling as standalone functions with GCC 5 I get 19 instructions 
vs. 17 for the asm, but the loop logic gets optimised out completely 
when ihl is a compile-time constant (e.g. inet_gro_receive())

Cheers,
Robin.

>
> 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@...adcom.com>
>> Signed-off-by: Robin Murphy <robin.murphy@....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 */
>