[Xen-devel] [PATCH 1/2] x86: improve NOP use for AMD CPUs

["Jan Beulich" <JBeulich@xxxxxxxx>]

For Fam10 and later AMD recommends using the "long" NOP forms. Re-write
the present Intel code into switch() statements and add AMD logic. This
at the same time brings us in line again with current Linux.

Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

--- a/xen/arch/x86/alternative.c
+++ b/xen/arch/x86/alternative.c
@@ -85,19 +85,34 @@ static int __init mask_nmi_callback(cons
 
 static void __init arch_init_ideal_nops(void)
 {
-    /*
-     * 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_vendor == X86_VENDOR_INTEL) &&
-         !(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 = p6_nops;
+    switch ( boot_cpu_data.x86_vendor )
+    {
+    case X86_VENDOR_INTEL:
+        /*
+         * 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 )
+            ideal_nops = p6_nops;
+        else
+            switch ( boot_cpu_data.x86_model )
+            {
+            case 0x0f ... 0x1b:
+            case 0x1d ... 0x25:
+            case 0x28 ... 0x2f:
+                break;
+            default:
+                ideal_nops = p6_nops;
+                break;
+            }
+        break;
+
+    case X86_VENDOR_AMD:
+        if ( boot_cpu_data.x86 > 0xf )
+            ideal_nops = p6_nops;
+        break;
+    }
 }
 
 /* Use this to add nops to a buffer, then text_poke the whole buffer. */




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