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Message-ID: <CAMzpN2hKr-=9sP=_VjGdsJDX5Pzdr9WAsSs77s_5yPJeqi728g@mail.gmail.com>
Date: Mon, 10 Oct 2022 23:51:33 -0400
From: Brian Gerst <brgerst@...il.com>
To: Xin Li <xin3.li@...el.com>
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
hpa@...or.com, peterz@...radead.org
Subject: Re: [PATCH v2 6/6] x86/gsseg: use the LKGS instruction if available
for load_gs_index()
On Mon, Oct 10, 2022 at 3:46 PM Xin Li <xin3.li@...el.com> wrote:
>
> From: "H. Peter Anvin (Intel)" <hpa@...or.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@...or.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Signed-off-by: Xin Li <xin3.li@...el.com>
> 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
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