x86/amd: Introduce is_zen{1,2}_uarch() predicates

We already have 3 cases using STIBP as a Zen1/2 heuristic, and are about to
introduce a 4th.  Wrap the heuristic into a pair of predicates rather than
opencoding it, and the explanation of the heuristic, at each usage site.

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

xen/arch/x86/cpu/amd.c

@@ -882,15 +882,13 @@ void amd_set_legacy_ssbd(bool enable)
  * non-branch instructions to be ignored.  It is to be set unilaterally in
  * newer microcode.
  *
- * This chickenbit is something unrelated on Zen1, and Zen1 vs Zen2 isn't a
- * simple model number comparison, so use STIBP as a heuristic to separate the
- * two uarches in Fam17h(AMD)/18h(Hygon).
+ * This chickenbit is something unrelated on Zen1.
  */
 void amd_init_spectral_chicken(void)
 {
 	uint64_t val, chickenbit = 1 << 1;
 
-	if (cpu_has_hypervisor || !boot_cpu_has(X86_FEATURE_AMD_STIBP))
+	if (cpu_has_hypervisor || !is_zen2_uarch())
 		return;
 
 	if (rdmsr_safe(MSR_AMD64_DE_CFG2, val) == 0 && !(val & chickenbit))
@@ -939,11 +937,8 @@ void amd_check_zenbleed(void)
 		 * With the Fam17h check above, most parts getting here are
 		 * Zen1.  They're not affected.  Assume Zen2 ones making it
 		 * here are affected regardless of microcode version.
-		 *
-		 * Zen1 vs Zen2 isn't a simple model number comparison, so use
-		 * STIBP as a heuristic to distinguish.
 		 */
-		if (!boot_cpu_has(X86_FEATURE_AMD_STIBP))
+		if (is_zen1_uarch())
 			return;
 		good_rev = ~0U;
 		break;
@@ -1298,12 +1293,7 @@ static int __init cf_check zen2_c6_errata_check(void)
 	 */
 	s_time_t delta;
 
-	/*
-	 * Zen1 vs Zen2 isn't a simple model number comparison, so use STIBP as
-	 * a heuristic to separate the two uarches in Fam17h.
-	 */
-	if (cpu_has_hypervisor || boot_cpu_data.x86 != 0x17 ||
-	    !boot_cpu_has(X86_FEATURE_AMD_STIBP))
+	if (cpu_has_hypervisor || boot_cpu_data.x86 != 0x17 || !is_zen2_uarch())
 		return 0;
 
 	/*

xen/arch/x86/include/asm/amd.h

@@ -140,6 +140,17 @@
                        AMD_MODEL_RANGE(0x11, 0x0, 0x0, 0xff, 0xf),	\
                        AMD_MODEL_RANGE(0x12, 0x0, 0x0, 0xff, 0xf))
 
+/*
+ * The Zen1 and Zen2 microarchitectures are implemented by AMD (Fam17h) and
+ * Hygon (Fam18h) but without simple model number rules.  Instead, use STIBP
+ * as a heuristic that distinguishes the two.
+ *
+ * The caller is required to perform the appropriate vendor/family checks
+ * first.
+ */
+#define is_zen1_uarch() (!boot_cpu_has(X86_FEATURE_AMD_STIBP))
+#define is_zen2_uarch()   boot_cpu_has(X86_FEATURE_AMD_STIBP)
+
 struct cpuinfo_x86;
 int cpu_has_amd_erratum(const struct cpuinfo_x86 *, int, ...);