linux-kernel - RE: [PATCH 6/6] x86/gsseg: use the LKGS instruction if available for load_gs

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Date:   Sat, 8 Oct 2022 05:31:20 +0000
From:   "Li, Xin3" <xin3.li@...el.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "hpa@...or.com" <hpa@...or.com>
Subject: RE: [PATCH 6/6] x86/gsseg: use the LKGS instruction if available for
 load_gs_index()

> Something like so perhaps?
> 
> 	asm_inline volatile("1:\n"
> 			    ALTERNATIVE("call asm_load_gs_index\n",
> 					LKGS_DI,
> 					X86_FEATURE_LKGS)
> 			    _ASM_EXTABLE_TYPE_REG(1b, 1b,
> EX_TYPE_ZERO_REG, %k[sel])
> 			    : ASM_CALL_CONSTRAINT
> 			    : [sel] "D" (sel)
> 			    : "memory", _ASM_AX);
> 
> 
> (completely untested, not even seen a compiler upclose)

It compiles (after adding "0x3e" to make it 5 bytes to match the size of "call asm_load_gs_index") and passes my fixup test, thanks!