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# T2 SDE: package/*/linux/hotfix-i686-detect-nopl.patch
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While removed upstream, we do not really care about decade old
broken commercial virtualizers. Check for real hw and potentially
use it.

--- linux-6.8/arch/x86/kernel/cpu/common.c.vanilla	2024-03-26 20:38:26.748240700 +0100
+++ linux-6.8/arch/x86/kernel/cpu/common.c	2024-03-26 20:43:34.183225214 +0100
@@ -1488,16 +1488,30 @@
 /*
  * The NOPL instruction is supposed to exist on all CPUs of family >= 6;
  * unfortunately, that's not true in practice because of early VIA
- * chips and (more importantly) broken virtualizers that are not easy
- * to detect. In the latter case it doesn't even *fail* reliably, so
- * probing for it doesn't even work. Disable it completely on 32-bit
- * unless we can find a reliable way to detect all the broken cases.
- * Enable it explicitly on 64-bit for non-constant inputs of cpu_has().
+ * chips and broken virtualizers. Instead, probe for NOPL directly by
+ * executing a NOPL instruction and see if we get #UD.
  */
 static void detect_nopl(void)
 {
 #ifdef CONFIG_X86_32
-	setup_clear_cpu_cap(X86_FEATURE_NOPL);
+	const u32 nopl_signature = 0x888c53b1; /* Random number */
+	u32 has_nopl = nopl_signature;
+
+ 	setup_clear_cpu_cap(X86_FEATURE_NOPL);
+	if (boot_cpu_data.x86 >= 6) {
+		asm volatile("\n"
+			     "1:      .byte 0x0f,0x1f,0xc0\n" /* nopl %eax */
+			     "2:\n"
+			     "        .section .fixup,\"ax\"\n"
+			     "3:      xor %0,%0\n"
+			     "        jmp 2b\n"
+			     "        .previous\n"
+			     _ASM_EXTABLE(1b,3b)
+			     : "+a" (has_nopl));
+
+		if (has_nopl == nopl_signature)
+			setup_force_cpu_cap(X86_FEATURE_NOPL);
+	}
 #else
 	setup_force_cpu_cap(X86_FEATURE_NOPL);
 #endif