Re: [PATCH 6/6] x86/gsseg: use the LKGS instruction if available for load_gs_index()

[H. Peter Anvin]

On October 7, 2022 7:47:09 AM PDT, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>On Thu, Oct 06, 2022 at 08:40:41AM -0700, Xin Li wrote:
>
>>  static inline void native_load_gs_index(unsigned int selector)
>>  {
>> +	u16 sel = selector;
>> +
>> +	/*
>> +	 * Note: the fixup is used for the LKGS instruction, but
>> +	 * it needs to be attached to the primary instruction sequence
>> +	 * as it isn't something that gets patched.
>> +	 *
>> +	 * %rax is provided to the assembly routine as a scratch
>> +	 * register.
>> +	 */
>> +	alternative_io("1: call asm_load_gs_index\n"
>> +		       ".pushsection \".fixup\",\"ax\"\n"
>> +		       "2:	xorl %k[sel], %k[sel]\n"
>> +		       "	jmp 1b\n"
>> +		       ".popsection\n"
>> +		       _ASM_EXTABLE(1b, 2b),
>> +		       _ASM_BYTES(0x3e) LKGS_DI,
>> +		       X86_FEATURE_LKGS,
>> +		       ASM_OUTPUT2([sel] "+D" (sel), ASM_CALL_CONSTRAINT),
>> +		       ASM_NO_INPUT_CLOBBER(_ASM_AX));
>>  }
>
>I'm very sure none of this was tested... the .fixup section hasn't
>existed for almost a year now.
>
>  e5eefda5aa51 ("x86: Remove .fixup section")

Xin, what did you use as the forward-porting baseline?