arm64: Implement optimised checksum routine

[Fred Kimmy]

[Bug Description]
Apparently there exist certain workloads which rely heavily on software
checksumming, for which the generic do_csum() implementation becomes a
significant bottleneck. Therefore let's give arm64 its own optimised
version - for ease of maintenance this foregoes assembly or intrisics,
and is thus not actually arm64-specific, but does rely heavily on C
idioms that translate well to the A64 ISA and the typical load/store
capabilities of most ARMv8 CPU cores.

The resulting increase in checksum throughput scales nicely with buffer
size, tending towards 4x for a small in-order core (Cortex-A53), and up
to 6x or more for an aggressive big core (Ampere eMAG).

[Steps to Reproduce]
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[Actual Results]

[Expected Results]

[Reproducibility]

[Additional information]
  (Firmware version, kernel version, affected hardware, etc. if required):

[Resolution]
v5.6 5777eaed566a arm64: Implement optimised checksum routine

[Andrew Cloke (andrew-cloke)]

This patch does not meet the criteria for SRU.

[Taihsiang Ho (tai271828)]

The commit is in focal hwe-5.8 kernel tree. tag: Ubuntu-hwe-5.8-5.8.0-25.26_20.04.1

[Ike Panhc (ikepanhc)]

Patch hits 20.04.2 HWE kernel.