RE: x86: should clear_user() have alternatives?

[David Laight]

From: Hugh Dickins
> Sent: 08 February 2022 05:46
> 
> I've noticed that clear_user() is slower than it need be:
> 
> dd if=/dev/zero of=/dev/null bs=1M count=1M
> 1099511627776 bytes (1.1 TB) copied, 45.9641 s, 23.9 GB/s
> whereas with the hacked patch below
> 1099511627776 bytes (1.1 TB) copied, 33.4 s, 32.9 GB/s
> 
> That was on some Intel machine: IIRC an AMD went faster.
> 
> It's because clear_user() lacks alternatives, and uses a
> nowadays suboptimal implementation; whereas clear_page()
> and copy_user() do support alternatives.
> 
...
> +SYM_FUNC_START(__clear_user)
> +	ASM_STAC
> +	movl %esi,%ecx
> +	xorq %rax,%rax
> +1:	rep stosb
> +2:	movl %ecx,%eax
> +	ASM_CLAC
> +	ret

You only want to even consider than version for long copies
(and possibly only for aligned ones).

The existing code (I've not quoted) does look sub-optimal though.
It should be easy to obtain a write every clock.
But I suspect the loop is too long.
The code gcc generates might even be better!

Note that for copies longer than 8 bytes 'odd' lengths can
be handled by a single misaligned write to the end of the buffer.
No need for a byte copy loop.

I've not experimented with misaligned writes - they might take two clocks.
So it might be worth aligning them - but they may not happen often
enough for it to be an overall gain.
Misaligned reads usually don't make any difference.

	David

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