x86: drop X86_FEATURE_3DNOW_ALT

Introducing an X86_FEATURE aliased value turns out to complicate automatic
processing of the feature list.  Drop X86_FEATURE_3DNOW_ALT and use
X86_FEATURE_PBE, extending the comment accordingly.

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

xen/arch/x86/cpu/amd.c

@@ -440,9 +440,12 @@ static void init_amd(struct cpuinfo_x86 *c)
 		wrmsrl(MSR_K7_HWCR, value);
 	}
 
-	/* Bit 31 in normal CPUID used for nonstandard 3DNow ID;
-	   3DNow is IDd by bit 31 in extended CPUID (1*32+31) anyway */
-	__clear_bit(X86_FEATURE_3DNOW_ALT, c->x86_capability);
+	/*
+	 * Some AMD CPUs duplicate the 3DNow bit in base and extended CPUID
+	 * leaves.  Unfortunately, this aliases PBE on Intel CPUs. Clobber the
+	 * alias, leaving 3DNow in the extended leaf.
+	 */
+	__clear_bit(X86_FEATURE_PBE, c->x86_capability);
 	
 	if (c->x86 == 0xf && c->x86_model < 0x14
 	    && cpu_has(c, X86_FEATURE_LAHF_LM)) {

xen/include/asm-x86/cpufeature.h

@@ -45,7 +45,6 @@
 #define X86_FEATURE_ACC		(0*32+29) /* Automatic clock control */
 #define X86_FEATURE_IA64	(0*32+30) /* IA-64 processor */
 #define X86_FEATURE_PBE		(0*32+31) /* Pending Break Enable */
-#define X86_FEATURE_3DNOW_ALT	(0*32+31) /* AMD nonstandard 3DNow (Aliases PBE) */
 
 /* AMD-defined CPU features, CPUID level 0x80000001, word 1 */
 /* Don't duplicate feature flags which are redundant with Intel! */