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

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Message-ID: <3F0DCCD0-2179-47E2-BC57-A948C0C91098@zytor.com>
Date:   Fri, 07 Oct 2022 11:49:54 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     "Li, Xin3" <xin3.li@...el.com>,
        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>
Subject: RE: [PATCH 6/6] x86/gsseg: use the LKGS instruction if available for load_gs_index()

On October 7, 2022 11:07:34 AM PDT, "Li, Xin3" <xin3.li@...el.com> wrote:
>> >> +	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?
>
>6.0 release, and my kernel dump shows me a fixup section is there, and a fixup section is created anyway if we do "pushsection "\.fixup\"".
>
>
>

Yeah. .fixup is really Just Another Text Section ™, so it is probably not surprising if it has crept back in.