Messages in this thread Patch in this message | | | From | Xin Li <> | Subject | [PATCH v2 6/6] x86/gsseg: use the LKGS instruction if available for load_gs_index() | Date | Mon, 10 Oct 2022 12:01:59 -0700 |
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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: Peter Zijlstra (Intel) <peterz@infradead.org> Signed-off-by: Xin Li <xin3.li@intel.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) + : "memory", _ASM_AX); } #endif /* CONFIG_X86_64 */ -- 2.34.1
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