x86/AMD: extend Zenbleed check to models "good" ucode isn't known for

Reportedly the AMD Custom APU 0405 found on SteamDeck, models 0x90 and
0x91, (quoting the respective Linux commit) is similarly affected. Put
another instance of our Zen1 vs Zen2 distinction checks in
amd_check_zenbleed(), forcing use of the chickenbit irrespective of
ucode version (building upon real hardware never surfacing a version of
0xffffffff).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
(cherry picked from commit 145a69c094)

xen/arch/x86/cpu/amd.c

@@ -800,10 +800,17 @@ void amd_check_zenbleed(void)
 	case 0xa0 ... 0xaf: good_rev = 0x08a00008; break;
 	default:
 		/*
-		 * With the Fam17h check above, parts getting here are Zen1.
-		 * They're not affected.
+		 * 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.
 		 */
-		return;
+		if (!boot_cpu_has(X86_FEATURE_AMD_STIBP))
+			return;
+		good_rev = ~0U;
+		break;
 	}
 
 	rdmsrl(MSR_AMD64_DE_CFG, val);