[PATCH] x86/alternatives: Switch AMD F15h and later to the P6 NOPs

[Borislav Petkov]

From: Borislav Petkov <bp@xxxxxxx>

Software optimization guides for both F15h and F16h cite those NOPs as
the optimal ones. A microbenchmark confirms that actually even older
families are better with the single-insn NOPs so switch to them for the
alternatives.

Cycles count below includes the loop overhead of the measurement but
that overhead is the same with all runs.

F10h, revE:
-----------
Running NOP tests, 1000 NOPs x 1000000 repetitions

K8:
                      90     288.212282 cycles
                   66 90     288.220840 cycles
                66 66 90     288.219447 cycles
             66 66 66 90     288.223204 cycles
          66 66 90 66 90     571.393424 cycles
       66 66 90 66 66 90     571.374919 cycles
    66 66 66 90 66 66 90     572.249281 cycles
 66 66 66 90 66 66 66 90     571.388651 cycles

P6:
                      90     288.214193 cycles
                   66 90     288.225550 cycles
                0f 1f 00     288.224441 cycles
             0f 1f 40 00     288.225030 cycles
          0f 1f 44 00 00     288.233558 cycles
       66 0f 1f 44 00 00     324.792342 cycles
    0f 1f 80 00 00 00 00     325.657462 cycles
 0f 1f 84 00 00 00 00 00     430.246643 cycles

F14h:
----
Running NOP tests, 1000 NOPs x 1000000 repetitions

K8:
                      90     510.404890 cycles
                   66 90     510.432117 cycles
                66 66 90     510.561858 cycles
             66 66 66 90     510.541865 cycles
          66 66 90 66 90    1014.192782 cycles
       66 66 90 66 66 90    1014.226546 cycles
    66 66 66 90 66 66 90    1014.334299 cycles
 66 66 66 90 66 66 66 90    1014.381205 cycles

P6:
                      90     510.436710 cycles
                   66 90     510.448229 cycles
                0f 1f 00     510.545100 cycles
             0f 1f 40 00     510.502792 cycles
          0f 1f 44 00 00     510.589517 cycles
       66 0f 1f 44 00 00     510.611462 cycles
    0f 1f 80 00 00 00 00     511.166794 cycles
 0f 1f 84 00 00 00 00 00     511.651641 cycles

F15h:
-----
Running NOP tests, 1000 NOPs x 1000000 repetitions

K8:
                      90     243.128396 cycles
                   66 90     243.129883 cycles
                66 66 90     243.131631 cycles
             66 66 66 90     242.499324 cycles
          66 66 90 66 90     481.829083 cycles
       66 66 90 66 66 90     481.884413 cycles
    66 66 66 90 66 66 90     481.851446 cycles
 66 66 66 90 66 66 66 90     481.409220 cycles

P6:
                      90     243.127026 cycles
                   66 90     243.130711 cycles
                0f 1f 00     243.122747 cycles
             0f 1f 40 00     242.497617 cycles
          0f 1f 44 00 00     245.354461 cycles
       66 0f 1f 44 00 00     361.930417 cycles
    0f 1f 80 00 00 00 00     362.844944 cycles
 0f 1f 84 00 00 00 00 00     480.514948 cycles

F16h:
-----
Running NOP tests, 1000 NOPs x 1000000 repetitions

K8:
                      90     507.793298 cycles
                   66 90     507.789636 cycles
                66 66 90     507.826490 cycles
             66 66 66 90     507.859075 cycles
          66 66 90 66 90    1008.663129 cycles
       66 66 90 66 66 90    1008.696259 cycles
    66 66 66 90 66 66 90    1008.692517 cycles
 66 66 66 90 66 66 66 90    1008.755399 cycles

P6:
                      90     507.795232 cycles
                   66 90     507.794761 cycles
                0f 1f 00     507.834901 cycles
             0f 1f 40 00     507.822629 cycles
          0f 1f 44 00 00     507.838493 cycles
       66 0f 1f 44 00 00     507.908597 cycles
    0f 1f 80 00 00 00 00     507.946417 cycles
 0f 1f 84 00 00 00 00 00     507.954960 cycles

Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Cc: Aravind Gopalakrishnan <aravind.gopalakrishnan@xxxxxxx>
---
 arch/x86/kernel/alternative.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index aef653193160..b0932c4341b3 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -227,6 +227,15 @@ void __init arch_init_ideal_nops(void)
 #endif
 		}
 		break;
+
+	case X86_VENDOR_AMD:
+		if (boot_cpu_data.x86 > 0xf) {
+			ideal_nops = p6_nops;
+			return;
+		}
+
+		/* fall through */
+
 	default:
 #ifdef CONFIG_X86_64
 		ideal_nops = k8_nops;
-- 
2.3.5

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