[PATCH v3 0/6] Enable LKGS instruction

[Xin Li]

LKGS instruction is introduced with Intel FRED (flexible return and event
delivery) specification https://cdrdv2.intel.com/v1/dl/getContent/678938.

LKGS is independent of FRED, so we enable it as a standalone CPU feature.

LKGS behaves like the MOV to GS instruction except that it loads the base
address into the IA32_KERNEL_GS_BASE MSR instead of the GS segment’s
descriptor cache, which is exactly what Linux kernel does to load user level
GS base.  Thus, with LKGS, there is no need to SWAPGS away from the kernel
GS base.

Changes since V2:
* add "" not to show "lkgs" in /proc/cpuinfo.
* mark DI as input and output (+D) as in V1, since the exception handler
  modifies it.

Changes since V1:
* use EX_TYPE_ZERO_REG instead of fixup code in the obsolete .fixup code
  section.
* add a comment that states the LKGS_DI macro will be repalced with "lkgs %di"
  once the binutils support LKGS instruction.

H. Peter Anvin (Intel) (6):
  x86/cpufeature: add cpu feature bit for LKGS
  x86/opcode: add LKGS instruction to x86-opcode-map
  x86/gsseg: make asm_load_gs_index() take an u16
  x86/gsseg: move local_irq_save/restore() into asm_load_gs_index()
  x86/gsseg: move load_gs_index() to its own header file
  x86/gsseg: use the LKGS instruction if available for load_gs_index()

 arch/x86/entry/entry_64.S                | 28 +++++++++---
 arch/x86/ia32/ia32_signal.c              |  1 +
 arch/x86/include/asm/cpufeatures.h       |  1 +
 arch/x86/include/asm/gsseg.h             | 56 ++++++++++++++++++++++++
 arch/x86/include/asm/mmu_context.h       |  1 +
 arch/x86/include/asm/special_insns.h     | 21 ---------
 arch/x86/kernel/paravirt.c               |  1 +
 arch/x86/kernel/tls.c                    |  1 +
 arch/x86/lib/x86-opcode-map.txt          |  1 +
 tools/arch/x86/include/asm/cpufeatures.h |  1 +
 tools/arch/x86/lib/x86-opcode-map.txt    |  1 +
 11 files changed, 86 insertions(+), 27 deletions(-)
 create mode 100644 arch/x86/include/asm/gsseg.h

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2.34.1