linux-kernel - Re: [PATCH v10 3/4] x86, mce: Add __mcsafe

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Message-ID: <20160210193905.GB29493@agluck-desk.sc.intel.com>
Date:	Wed, 10 Feb 2016 11:39:05 -0800
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Borislav Petkov <bp@...en8.de>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andy Lutomirski <luto@...nel.org>,
	Dan Williams <dan.j.williams@...el.com>, elliott@....com,
	Brian Gerst <brgerst@...il.com>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-nvdimm@...1.01.org, x86@...nel.org
Subject: Re: [PATCH v10 3/4] x86, mce: Add __mcsafe_copy()

On Wed, Feb 10, 2016 at 11:58:43AM +0100, Borislav Petkov wrote:
> But one could take out that function do some microbenchmarking with
> different sizes and once with the current version and once with the
> pushes and pops of r1[2-5] to see where the breakeven is.

On a 4K page copy from a source address that isn't in the
cache I see all sorts of answers.

On my desktop (i7-3960X) it is ~50 cycles slower to push and pop the four
registers.

On my latest Xeon - I can't post benchmarks ... but also a bit slower.

On an older Xeon it is a few cycles faster (but even though I'm
looking at the median of 10,000 runs I see more run-to-run variation
that I see difference between register choices.

Here's what I tested:

	push %r12
	push %r13
	push %r14
	push %r15

	/* Loop copying whole cache lines */
1:	movq (%rsi),%r8
2:	movq 1*8(%rsi),%r9
3:	movq 2*8(%rsi),%r10
4:	movq 3*8(%rsi),%r11
9:	movq 4*8(%rsi),%r12
10:	movq 5*8(%rsi),%r13
11:	movq 6*8(%rsi),%r14
12:	movq 7*8(%rsi),%r15
	movq %r8,(%rdi)
	movq %r9,1*8(%rdi)
	movq %r10,2*8(%rdi)
	movq %r11,3*8(%rdi)
	movq %r12,4*8(%rdi)
	movq %r13,5*8(%rdi)
	movq %r14,6*8(%rdi)
	movq %r15,7*8(%rdi)
	leaq 64(%rsi),%rsi
	leaq 64(%rdi),%rdi
	decl %ecx
	jnz 1b

	pop %r15
	pop %r14
	pop %r13
	pop %r12
-Tony