x86: improve NOP use for AMD CPUs

For Fam10 and later AMD recommends using the "long" NOP forms. Re-write
the present Intel code into switch() statements and add AMD logic.

Default to "long" forms (which all 64-bit CPUs are supposed to
recognize), overriding to the K8 flavor on those few (older) CPUs.

This at the same time brings us in line again in this regard with
current Linux.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

xen/arch/x86/alternative.c

@@ -76,7 +76,7 @@ static const unsigned char * const p6_nops[ASM_NOP_MAX+1] init_or_livepatch_cons
 };
 #endif
 
-static const unsigned char * const *ideal_nops init_or_livepatch_data = k8_nops;
+static const unsigned char * const *ideal_nops init_or_livepatch_data = p6_nops;
 
 static int __init mask_nmi_callback(const struct cpu_user_regs *regs, int cpu)
 {
@@ -85,19 +85,32 @@ static int __init mask_nmi_callback(const struct cpu_user_regs *regs, int cpu)
 
 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 )
+            break;
+
+        switch ( boot_cpu_data.x86_model )
+        {
+        case 0x0f ... 0x1b:
+        case 0x1d ... 0x25:
+        case 0x28 ... 0x2f:
+            ideal_nops = k8_nops;
+            break;
+        }
+        break;
+
+    case X86_VENDOR_AMD:
+        if ( boot_cpu_data.x86 <= 0xf )
+            ideal_nops = k8_nops;
+        break;
+    }
 }
 
 /* Use this to add nops to a buffer, then text_poke the whole buffer. */