[PATCH 25/24] x86/traps: Rewrite native_load_gs_index in C code

[Lai Jiangshan <jiangshanlai@gmail.com>]

From: Lai Jiangshan <laijs@linux.alibaba.com>

There is no constrain/limition to force native_load_gs_index() to be in
ASM code.

Signed-off-by: Lai Jiangshan <laijs@linux.alibaba.com>
---
 arch/x86/entry/entry_64.S            | 36 ----------------------------
 arch/x86/entry/traps.c               | 25 +++++++++++++++++++
 arch/x86/include/asm/special_insns.h | 11 +--------
 arch/x86/mm/extable.c                | 15 ++++++++++++
 4 files changed, 41 insertions(+), 46 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 2d17fca1f503..16ee481e9b5f 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -659,42 +659,6 @@ native_irq_return_ldt:
 SYM_CODE_END(common_interrupt_return)
 _ASM_NOKPROBE(common_interrupt_return)
 
-/*
- * Reload gs selector with exception handling
- * edi:  new selector
- *
- * Is in entry.text as it shouldn't be instrumented.
- */
-SYM_FUNC_START(asm_load_gs_index)
-	FRAME_BEGIN
-	swapgs
-SYM_INNER_LABEL(asm_load_gs_index_gs_change, SYM_L_GLOBAL)
-.Lgs_change:
-	movl	%edi, %gs
-2:	ALTERNATIVE "", "mfence", X86_BUG_SWAPGS_FENCE
-	swapgs
-	FRAME_END
-	ret
-SYM_FUNC_END(asm_load_gs_index)
-EXPORT_SYMBOL(asm_load_gs_index)
-
-	_ASM_EXTABLE(.Lgs_change, .Lbad_gs)
-	.section .fixup, "ax"
-	/* running with kernelgs */
-SYM_CODE_START_LOCAL_NOALIGN(.Lbad_gs)
-	swapgs					/* switch back to user gs */
-.macro ZAP_GS
-	/* This can't be a string because the preprocessor needs to see it. */
-	movl $__USER_DS, %eax
-	movl %eax, %gs
-.endm
-	ALTERNATIVE "", "ZAP_GS", X86_BUG_NULL_SEG
-	xorl	%eax, %eax
-	movl	%eax, %gs
-	jmp	2b
-SYM_CODE_END(.Lbad_gs)
-	.previous
-
 #ifdef CONFIG_XEN_PV
 /*
  * A note on the "critical region" in our callback handler.
diff --git a/arch/x86/entry/traps.c b/arch/x86/entry/traps.c
index 52511db6baa6..2a17d74569fd 100644
--- a/arch/x86/entry/traps.c
+++ b/arch/x86/entry/traps.c
@@ -679,6 +679,31 @@ DEFINE_IDTENTRY_RAW(exc_int3)
 }
 
 #ifdef CONFIG_X86_64
+
+/*
+ * Reload gs selector with exception handling
+ * selector:  new selector
+ *
+ * Is noinstr as it shouldn't be instrumented.
+ */
+noinstr void native_load_gs_index(unsigned int selector)
+{
+	unsigned long flags;
+
+	local_irq_save(flags);
+	native_swapgs();
+	asm volatile(
+		".global asm_load_gs_index_gs_change \n"
+		"asm_load_gs_index_gs_change: \n"
+		"1: movl %0, %%gs \n"
+		"   swapgs \n"
+		"2: \n"
+		_ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_clear_gs)
+		:: "r" (selector) : "memory");
+	alternative("", "mfence", X86_BUG_SWAPGS_FENCE);
+	local_irq_restore(flags);
+}
+
 /*
  * Help handler running on a per-cpu (IST or entry trampoline) stack
  * to switch to the normal thread stack if the interrupted code was in
diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
index 058995bb153c..c5d74ebce527 100644
--- a/arch/x86/include/asm/special_insns.h
+++ b/arch/x86/include/asm/special_insns.h
@@ -120,16 +120,7 @@ static inline void native_wbinvd(void)
 	asm volatile("wbinvd": : :"memory");
 }
 
-extern asmlinkage void asm_load_gs_index(unsigned int selector);
-
-static inline void native_load_gs_index(unsigned int selector)
-{
-	unsigned long flags;
-
-	local_irq_save(flags);
-	asm_load_gs_index(selector);
-	local_irq_restore(flags);
-}
+extern asmlinkage void native_load_gs_index(unsigned int selector);
 
 static inline unsigned long __read_cr4(void)
 {
diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index e1664e9f969c..695a580a652a 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -138,6 +138,21 @@ __visible bool ex_handler_clear_fs(const struct exception_table_entry *fixup,
 }
 EXPORT_SYMBOL(ex_handler_clear_fs);
 
+__visible bool ex_handler_clear_gs(const struct exception_table_entry *fixup,
+				   struct pt_regs *regs, int trapnr,
+				   unsigned long error_code,
+				   unsigned long fault_addr)
+{
+	/*
+	 * The following native_load_gs_index() should not fault, otherwise
+	 * it would be infinite recursion.
+	 */
+	if (static_cpu_has(X86_BUG_NULL_SEG))
+		native_load_gs_index(__USER_DS);
+	native_load_gs_index(0);
+	return ex_handler_default(fixup, regs, trapnr, error_code, fault_addr);
+}
+
 enum handler_type ex_get_fault_handler_type(unsigned long ip)
 {
 	const struct exception_table_entry *e;
-- 
2.19.1.6.gb485710b