Re: [PATCH v2 6/6] x86/gsseg: use the LKGS instruction if available for load_gs_index()

[Brian Gerst]

On Mon, Oct 10, 2022 at 3:46 PM Xin Li <xin3.li@xxxxxxxxx> wrote:
>
> From: "H. Peter Anvin (Intel)" <hpa@xxxxxxxxx>
>
> 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@xxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Xin Li <xin3.li@xxxxxxxxx>
> link: https://lkml.org/lkml/2022/10/7/352
> link: https://lkml.org/lkml/2022/10/7/373
> ---
>  arch/x86/include/asm/gsseg.h | 27 ++++++++++++++++++++++++++-
>  1 file changed, 26 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/gsseg.h b/arch/x86/include/asm/gsseg.h
> index 5e3b56a17098..4aaef7a1d68f 100644
> --- a/arch/x86/include/asm/gsseg.h
> +++ b/arch/x86/include/asm/gsseg.h
> @@ -3,15 +3,40 @@
>  #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);
>
> +/* Replace with "lkgs %di" once binutils support LKGS instruction */
> +#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.
> +        */
> +       asm_inline volatile("1:\n"
> +                           ALTERNATIVE("call asm_load_gs_index\n",
> +                                       _ASM_BYTES(0x3e) LKGS_DI,
> +                                       X86_FEATURE_LKGS)
> +                           _ASM_EXTABLE_TYPE_REG(1b, 1b, EX_TYPE_ZERO_REG, %k[sel])
> +                           : ASM_CALL_CONSTRAINT
> +                           : [sel] "D" (sel)

DI needs to be marked as input and output (+D), since the exception
handler modifies it.

> +                           : "memory", _ASM_AX);

_ASM_AX is only needed for code that is used by both 32 and 64-bit.
Since this is 64-bit only, "rax" would be appropriate.

--
Brian Gerst