package/kernel/linux/hotfix-i686-detect-nopl.patch

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   2 # T2 SDE: package/*/linux/hotfix-i686-detect-nopl.patch
   3 # Copyright (C) 2024 The T2 SDE Project
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  10 # as defined at http://www.opensource.org/ (e.g. BSD, X11) or under the terms
  11 # of the GNU General Public License version 2 as used by the T2 SDE.
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  13
  14 While removed upstream, we do not really care about decade old
  15 broken commercial virtualizers. Check for real hw and potentially
  16 use it.
  17
  18 --- linux-6.8/arch/x86/kernel/cpu/common.c.vanilla      2024-03-26 20:38:26.748240700 +0100
  19 +++ linux-6.8/arch/x86/kernel/cpu/common.c      2024-03-26 20:43:34.183225214 +0100
  20 @@ -1488,16 +1488,30 @@
  21  /*
  22   * The NOPL instruction is supposed to exist on all CPUs of family >= 6;
  23   * unfortunately, that's not true in practice because of early VIA
  24 - * chips and (more importantly) broken virtualizers that are not easy
  25 - * to detect. In the latter case it doesn't even *fail* reliably, so
  26 - * probing for it doesn't even work. Disable it completely on 32-bit
  27 - * unless we can find a reliable way to detect all the broken cases.
  28 - * Enable it explicitly on 64-bit for non-constant inputs of cpu_has().
  29 + * chips and broken virtualizers. Instead, probe for NOPL directly by
  30 + * executing a NOPL instruction and see if we get #UD.
  31   */
  32  static void detect_nopl(void)
  33  {
  34  #ifdef CONFIG_X86_32
  35 -       setup_clear_cpu_cap(X86_FEATURE_NOPL);
  36 +       const u32 nopl_signature = 0x888c53b1; /* Random number */
  37 +       u32 has_nopl = nopl_signature;
  38 +
  39 +       setup_clear_cpu_cap(X86_FEATURE_NOPL);
  40 +       if (boot_cpu_data.x86 >= 6) {
  41 +               asm volatile("\n"
  42 +                            "1:      .byte 0x0f,0x1f,0xc0\n" /* nopl %eax */
  43 +                            "2:\n"
  44 +                            "        .section .fixup,\"ax\"\n"
  45 +                            "3:      xor %0,%0\n"
  46 +                            "        jmp 2b\n"
  47 +                            "        .previous\n"
  48 +                            _ASM_EXTABLE(1b,3b)
  49 +                            : "+a" (has_nopl));
  50 +
  51 +               if (has_nopl == nopl_signature)
  52 +                       setup_force_cpu_cap(X86_FEATURE_NOPL);
  53 +       }
  54  #else
  55         setup_force_cpu_cap(X86_FEATURE_NOPL);
  56  #endif