Re: [PATCH 6/6] x86/gsseg: use the LKGS instruction if available for load_gs_index()
From: H. Peter Anvin
Date: Fri Oct 07 2022 - 13:45:54 EST
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?