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Subject: Re: RFC: Petition Intel/AMD to add POPF_IF insn
To: Linus Torvalds <torvalds@linux-foundation.org>
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From: Denys Vlasenko <dvlasenk@redhat.com>
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Date: Thu, 18 Aug 2016 19:47:56 +0200
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On 08/18/2016 07:24 PM, Linus Torvalds wrote:
> That said, your numbers really aren't very convincing. If popf really
> is just 10 cycles on modern Intel hardware, it's already fast enough
> that I really don't think it matters.

It's 20 cycles. I was wrong in my email, I forgot that the insn count
also counts "push %ebx" insns.

Since I already made a mistake, let me double-check.

200 million iterations of this loop execute under 17 seconds:

   400100:	b8 00 c2 eb 0b       	mov    $0xbebc200,%eax # 1000*1000*1000
   400105:	9c                   	pushfq
   400106:	5b                   	pop    %rbx
   400107:	90                   	nop
....
0000000000400140 <loop>:
   400140:	53                   	push   %rbx
   400141:	9d                   	popfq
   400142:	53                   	push   %rbx
   400143:	9d                   	popfq
   400144:	53                   	push   %rbx
   400145:	9d                   	popfq
   400146:	53                   	push   %rbx
   400147:	9d                   	popfq
   400148:	53                   	push   %rbx
   400149:	9d                   	popfq
   40014a:	53                   	push   %rbx
   40014b:	9d                   	popfq
   40014c:	53                   	push   %rbx
   40014d:	9d                   	popfq
   40014e:	53                   	push   %rbx
   40014f:	9d                   	popfq
   400150:	53                   	push   %rbx
   400151:	9d                   	popfq
   400152:	53                   	push   %rbx
   400153:	9d                   	popfq
   400154:	53                   	push   %rbx
   400155:	9d                   	popfq
   400156:	53                   	push   %rbx
   400157:	9d                   	popfq
   400158:	53                   	push   %rbx
   400159:	9d                   	popfq
   40015a:	53                   	push   %rbx
   40015b:	9d                   	popfq
   40015c:	ff c8                	dec    %eax
   40015e:	75 e0                	jne    400140 <loop>

The loop is exactly 32 bytes, aligned.
There are 14 POPFs. Other insns are very fast.

No perf, just "time taskset 1 ./test".
My CPU frequency hovers around 3500 MHz when loaded.

17 seconds is 17*3500 million cycles.
17*3500 million cycles / 200*14 million cycles = 21.25

Thus, one POPF in CPL3 is ~20 cycles on Skylake.