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

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Date:   Fri, 7 Oct 2022 22:23:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "H. Peter Anvin" <hpa@...or.com>
Cc:     "Li, Xin3" <xin3.li@...el.com>,
        "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 Fri, Oct 07, 2022 at 01:03:12PM -0700, H. Peter Anvin wrote:
> 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?

Whatever for? I know the robots report this warning because there was
one from the KVM cross-merge when the .fixup removal went in. It got
reported and fixed and that was the last of it.

Anyway; try:

  $ git grep "\.fixup" arch/x86/

there isn't a single usage.

Andrew Cooper suggested upgrading the orphan section warning to a hard
link error, orphan sections are bad regardless.