RE: [PATCH] x86: only use ERMS for user copies for larger sizes

[David Laight]

From: Denys Vlasenko
> Sent: 21 November 2018 13:44
...
> I also tested this while working for string ops code in musl.
> 
> I think at least 128 bytes would be the minimum where "REP insn"
> are more efficient. In my testing, it's more like 256 bytes...

What happens for misaligned copies?
I had a feeling that the ERMS 'reb movsb' code used some kind
of barrel shifter in that case.

The other problem with the ERMS copy is that it gets used
for copy_to/from_io() - and the 'rep movsb' on uncached
locations has to do byte copies.
Byte reads on PCIe are really horrid.

	David

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