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From:   Brian Gerst <brgerst@gmail.com>
Date:   Fri, 7 Oct 2022 12:18:45 -0400
Message-ID: <CAMzpN2iccL5kNa2UaBXppiLnoNWrpwJd74+uBrB_63N0F5F5Xg@mail.gmail.com>
Subject: Re: [PATCH 6/6] x86/gsseg: use the LKGS instruction if available for load_gs_index()
To:     Xin Li <xin3.li@intel.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        hpa@zytor.com
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On Thu, Oct 6, 2022 at 12:19 PM Xin Li <xin3.li@intel.com> wrote:
>
> From: "H. Peter Anvin (Intel)" <hpa@zytor.com>
>
> The LKGS instruction atomically loads a segment descriptor into the
> %gs descriptor registers, *except* that %gs.base is unchanged, and the
> base is instead loaded into MSR_IA32_KERNEL_GS_BASE, which is exactly
> what we want this function to do.
>
> Signed-off-by: H. Peter Anvin (Intel) <hpa@zytor.com>
> Signed-off-by: Xin Li <xin3.li@intel.com>
> ---
>  arch/x86/include/asm/gsseg.h | 28 +++++++++++++++++++++++++++-
>  1 file changed, 27 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/gsseg.h b/arch/x86/include/asm/gsseg.h
> index 5e3b56a17098..b8a6a98d88b8 100644
> --- a/arch/x86/include/asm/gsseg.h
> +++ b/arch/x86/include/asm/gsseg.h
> @@ -3,15 +3,41 @@
>  #define _ASM_X86_GSSEG_H
>
>  #include <linux/types.h>
> +
> +#include <asm/asm.h>
> +#include <asm/cpufeature.h>
> +#include <asm/alternative.h>
>  #include <asm/processor.h>
> +#include <asm/nops.h>
>
>  #ifdef CONFIG_X86_64
>
>  extern asmlinkage void asm_load_gs_index(u16 selector);
>
> +#define LKGS_DI        _ASM_BYTES(0xf2,0x0f,0x00,0xf7)
> +
>  static inline void native_load_gs_index(unsigned int selector)
>  {
> -       asm_load_gs_index(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));
>  }
>
>  #endif /* CONFIG_X86_64 */
> --
> 2.34.1

There are not that many call sites, so using something like this
(incorporating Peter Z's suggestion for the exception handler) would
be better from a code readability perspective vs. a tiny increase in
code size.

        if (static_cpu_has(X86_FEATURE_LKGS))
                asm volatile("1: " LKGS_DI
                             _ASM_EXTABLE_TYPE_REG(1b, 1b,
EX_TYPE_ZERO_REG, %k[sel])
                             : [sel] "+D" (sel) : : "memory");
        else
                asm_load_gs_index(sel);

--
Brian Gerst