Re: [PATCH v2 09/12] x86/msr: Use the alternatives mechanism for WRMSR

[Peter Zijlstra]

On Tue, Sep 30, 2025 at 05:42:13PM +0200, Jürgen Groß wrote:

> Could it be that the labels should be local ones?

I already tried 's/#pfx/".L" #pfx/g' and that made no difference.

> What does the failure look like?

Its complaining about label re-definitions.

$ make O=defconfig-build/ arch/x86/kernel/alternative.o
../arch/x86/include/asm/cpufeature.h: Assembler messages:
../arch/x86/include/asm/cpufeature.h:102: Error: symbol `.L__UNIQUE_ID_altinstr_67_begin' is already defined
../arch/x86/include/asm/cpufeature.h:104: Error: symbol `.L__UNIQUE_ID_altinstr_66_begin' is already defined
../arch/x86/include/asm/cpufeature.h:106: Error: symbol `.L__UNIQUE_ID_altinstr_66_pad' is already defined
../arch/x86/include/asm/cpufeature.h:109: Error: symbol `.L__UNIQUE_ID_altinstr_66_end' is already defined
../arch/x86/include/asm/cpufeature.h:119: Error: symbol `.L__UNIQUE_ID_altinstr_66_alt_begin' is already defined
../arch/x86/include/asm/cpufeature.h:121: Error: symbol `.L__UNIQUE_ID_altinstr_66_alt_end' is already defined
../arch/x86/include/asm/cpufeature.h:124: Error: symbol `.L__UNIQUE_ID_altinstr_67_pad' is already defined
../arch/x86/include/asm/cpufeature.h:127: Error: symbol `.L__UNIQUE_ID_altinstr_67_end' is already defined
../arch/x86/include/asm/cpufeature.h:137: Error: symbol `.L__UNIQUE_ID_altinstr_67_alt_begin' is already defined
../arch/x86/include/asm/cpufeature.h:139: Error: symbol `.L__UNIQUE_ID_altinstr_67_alt_end' is already defined
../arch/x86/include/asm/cpufeature.h:102: Error: symbol `.L__UNIQUE_ID_altinstr_67_begin' is already defined
../arch/x86/include/asm/cpufeature.h:104: Error: symbol `.L__UNIQUE_ID_altinstr_66_begin' is already defined
../arch/x86/include/asm/cpufeature.h:106: Error: symbol `.L__UNIQUE_ID_altinstr_66_pad' is already defined
../arch/x86/include/asm/cpufeature.h:109: Error: symbol `.L__UNIQUE_ID_altinstr_66_end' is already defined
../arch/x86/include/asm/cpufeature.h:119: Error: symbol `.L__UNIQUE_ID_altinstr_66_alt_begin' is already defined
../arch/x86/include/asm/cpufeature.h:121: Error: symbol `.L__UNIQUE_ID_altinstr_66_alt_end' is already defined
../arch/x86/include/asm/cpufeature.h:124: Error: symbol `.L__UNIQUE_ID_altinstr_67_pad' is already defined
../arch/x86/include/asm/cpufeature.h:127: Error: symbol `.L__UNIQUE_ID_altinstr_67_end' is already defined
../arch/x86/include/asm/cpufeature.h:137: Error: symbol `.L__UNIQUE_ID_altinstr_67_alt_begin' is already defined
../arch/x86/include/asm/cpufeature.h:139: Error: symbol `.L__UNIQUE_ID_altinstr_67_alt_end' is already defined
../arch/x86/include/asm/cpufeature.h:102: Error: symbol `.L__UNIQUE_ID_altinstr_67_begin' is already defined
../arch/x86/include/asm/cpufeature.h:104: Error: symbol `.L__UNIQUE_ID_altinstr_66_begin' is already defined
../arch/x86/include/asm/cpufeature.h:106: Error: symbol `.L__UNIQUE_ID_altinstr_66_pad' is already defined
../arch/x86/include/asm/cpufeature.h:109: Error: symbol `.L__UNIQUE_ID_altinstr_66_end' is already defined
../arch/x86/include/asm/cpufeature.h:119: Error: symbol `.L__UNIQUE_ID_altinstr_66_alt_begin' is already defined
../arch/x86/include/asm/cpufeature.h:121: Error: symbol `.L__UNIQUE_ID_altinstr_66_alt_end' is already defined
../arch/x86/include/asm/cpufeature.h:124: Error: symbol `.L__UNIQUE_ID_altinstr_67_pad' is already defined
../arch/x86/include/asm/cpufeature.h:127: Error: symbol `.L__UNIQUE_ID_altinstr_67_end' is already defined
...

That specific one is this:

  static inline __attribute__((__gnu_inline__)) __attribute__((__unused__)) __attribute__((no_instrument_function)) __attribute__((__always_inline__)) bool _static_cpu_has(u16 bit)
  {
   asm goto("# ALT: oldinstr\n" ".L" "__UNIQUE_ID_altinstr_67" "_begin:\n\t" "# ALT: oldinstr\n" ".L" "__UNIQUE_ID_altinstr_66" "_begin:\n\t" "jmp 6f" "\n" ".L" "__UNIQUE_ID_altinstr_66" "_pad:\n" "# ALT: padding\n" ".skip -(((" ".L" "_   _UNIQUE_ID_altinstr_66" "_alt_end - " ".L" "__UNIQUE_ID_altinstr_66" "_alt_begin" ")-(" ".L" "__UNIQUE_ID_altinstr_66" "_pad - " ".L" "__UNIQUE_ID_altinstr_66" "_begin" ")) > 0) * " "((" ".L" "__UNIQUE_ID_altinstr_66" "_alt_end - " "   .L" "__UNIQUE_ID_altinstr_66" "_alt_begin" ")-(" ".L" "__UNIQUE_ID_altinstr_66" "_pad - " ".L" "__UNIQUE_ID_altinstr_66" "_begin" ")),0x90\n" ".L" "__UNIQUE_ID_altinstr_66" "_end:\n" ".pushsection .altinstructions,\"a\"\n" " .long "    ".L" "__UNIQUE_ID_altinstr_66" "_begin - .\n" " .long " ".L" "__UNIQUE_ID_altinstr_66" "_alt_begin - .\n" " .4byte " "( 3*32+21)" "\n" " .byte " ".L" "__UNIQUE_ID_altinstr_66" "_end - " ".L" "__UNIQUE_ID_altinstr_66" "_begin" "\n" "    .byte " ".L" "__UNIQUE_ID_altinstr_66" "_alt_end - " ".L" "__UNIQUE_ID_altinstr_66" "_alt_begin" "\n" ".popsection\n" ".pushsection .altinstr_replacement, \"ax\"\n" "# ALT: replacement\n" ".L" "__UNIQUE_ID_altinstr_66" "_alt_begin:\n   \t" "jmp %l[t_no]" "\n" ".L" "__UNIQUE_ID_altinstr_66" "_alt_end:\n" ".popsection\n" "\n" ".L" "__UNIQUE_ID_altinstr_67" "_pad:\n" "# ALT: padding\n" ".skip -(((" ".L" "__UNIQUE_ID_altinstr_67" "_alt_end - " ".L" "__UNIQUE_ID_altinst   r_67" "_alt_begin" ")-(" ".L" "__UNIQUE_ID_altinstr_67" "_pad - " ".L" "__UNIQUE_ID_altinstr_67" "_begin" ")) > 0) * " "((" ".L" "__UNIQUE_ID_altinstr_67" "_alt_end - " ".L" "__UNIQUE_ID_altinstr_67" "_alt_begin" ")-(" ".L" "__UNIQUE   _ID_altinstr_67" "_pad - " ".L" "__UNIQUE_ID_altinstr_67" "_begin" ")),0x90\n" ".L" "__UNIQUE_ID_altinstr_67" "_end:\n" ".pushsection .altinstructions,\"a\"\n" " .long " ".L" "__UNIQUE_ID_altinstr_67" "_begin - .\n" " .long " ".L" "_   _UNIQUE_ID_altinstr_67" "_alt_begin - .\n" " .4byte " "%c[feature]" "\n" " .byte " ".L" "__UNIQUE_ID_altinstr_67" "_end - " ".L" "__UNIQUE_ID_altinstr_67" "_begin" "\n" " .byte " ".L" "__UNIQUE_ID_altinstr_67" "_alt_end - " ".L" "__U   NIQUE_ID_altinstr_67" "_alt_begin" "\n" ".popsection\n" ".pushsection .altinstr_replacement, \"ax\"\n" "# ALT: replacement\n" ".L" "__UNIQUE_ID_altinstr_67" "_alt_begin:\n\t" "" "\n" ".L" "__UNIQUE_ID_altinstr_67" "_alt_end:\n" ".pop   section\n"
    ".pushsection .altinstr_aux,\"ax\"\n"
    "6:\n"
    " testb %[bitnum], %a[cap_byte]\n"
    " jnz %l[t_yes]\n"
    " jmp %l[t_no]\n"
    ".popsection\n"
     : : [feature] "i" (bit),
         [bitnum] "i" (1 << (bit & 7)),
         [cap_byte] "i" (&((const char *)boot_cpu_data.x86_capability)[bit >> 3])
     : : t_yes, t_no);
  t_yes:
   return true;
  t_no:
   return false;
  }

What I'm thinking is happening is that the __always_inline__ is causing
multiple exact copies of the thing.

Let me see how terrible it all ends up when using as macros