x86, cpu: Change NOP selection for certain Intel CPUs

Due to a decoder implementation quirk, some specific Intel CPUs
actually perform better with the "k8_nops" than with the
SDM-recommended NOPs.  For runtime-selected NOPs, if we detect those
specific CPUs then use the k8_nops instead of the ones we would
normally use.

Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>
Cc: Tejun Heo <tj@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jason Baron <jbaron@redhat.com>
Link: http://lkml.kernel.org/r/1303166160-10315-4-git-send-email-hpa@linux.intel.com

1 files changed, 13 insertions, 1 deletions

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index 846f61eb89c1..c0501ea6b634 100644
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -199,7 +199,19 @@ void __init arch_init_ideal_nops(void)
 {
         switch (boot_cpu_data.x86_vendor) {
         case X86_VENDOR_INTEL:
-                if (boot_cpu_has(X86_FEATURE_NOPL)) {
+                /*
+                 * Due to a decoder implementation quirk, some
+                 * specific Intel CPUs actually perform better with
+                 * the "k8_nops" than with the SDM-recommended NOPs.
+                 */
+                if (boot_cpu_data.x86 == 6 &&
+                    boot_cpu_data.x86_model >= 0x0f &&
+                    boot_cpu_data.x86_model != 0x1c &&
+                    boot_cpu_data.x86_model != 0x26 &&
+                    boot_cpu_data.x86_model != 0x27 &&
+                    boot_cpu_data.x86_model < 0x30) {
+                        ideal_nops = k8_nops;
+                } else if (boot_cpu_has(X86_FEATURE_NOPL)) {
                            ideal_nops = p6_nops;
                 } else {
 #ifdef CONFIG_X86_64