From: Al Viro
> Sent: 22 July 2020 18:39
> I would love to see your patch, anyway, along with the testcases and performance
> comparison.

See attached program.
Compile and run (as root): csum_iov 1

Unpatched (as shipped) 16 vectors of 1 byte take ~430 clocks on my haswell cpu.
With dsl_patch defined they take ~393.

The maximum throughput is ~1.16 clocks/word for 16 vectors of 1k.
For longer vectors the data gets lost from the cache between the iterations.

On an older Ivy Bridge cpu it never goes faster than 2 clocks/word.
(Due to the implementation of ADC.)

The absolute limit is 1 clock/word - limited by the memory write.
I suspect that is achievable on Haswell with much less loop unrolling.

I had to replace the ror32() with __builtin_bswap32().
The kernel object do contain the 'ror' instruction - even though I
didn't find the asm for it.

	David

-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)