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

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Message-ID: <80AC2E90-D842-4EA2-A413-3CC5CFF088BC@zytor.com>
Date:   Fri, 07 Oct 2022 13:03:12 -0700
From:   "H. Peter Anvin" <hpa@...or.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        "Li, Xin3" <xin3.li@...el.com>
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 12:24:13 PM PDT, Peter Zijlstra <peterz@...radead.org> wrote:
>On Fri, Oct 07, 2022 at 06:01:06PM +0000, Li, Xin3 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.
>> 
>> Weird, did you ever check a kernel dump?
>
>$ readelf -WS defconfig-build/vmlinux | grep fixup
>[ 5] .pci_fixup        PROGBITS        ffffffff826a5350 18a5350 003570 00   A  0   0 16
>[ 6] .rela.pci_fixup   RELA            0000000000000000 360c388 005028 18   I 60   5  8
>
>In fact, when I add one I get:
>
>ld: warning: orphan section `.fixup' from `arch/x86/kernel/traps.o' being placed in section `.fixup'

Perhaps the two of you need to compare confugurations?