[RFC PATCH V2 5/7] x86/entry: don't shift stack on #DB

[Lai Jiangshan]

debug_enter() will disable #DB, there should be no recursive #DB.

Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: x86@xxxxxxxxxx
Signed-off-by: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
---
 arch/x86/entry/entry_64.S        | 17 -----------------
 arch/x86/kernel/asm-offsets_64.c |  1 -
 2 files changed, 18 deletions(-)

diff --git a/arch/x86/entry/entry_64.S b/arch/x86/entry/entry_64.S
index 265ff97b3961..8ecaeee53653 100644
--- a/arch/x86/entry/entry_64.S
+++ b/arch/x86/entry/entry_64.S
@@ -396,11 +396,6 @@ SYM_CODE_END(\asmsym)
 	idtentry \vector asm_\cfunc \cfunc has_error_code=0
 .endm
 
-/*
- * MCE and DB exceptions
- */
-#define CPU_TSS_IST(x) PER_CPU_VAR(cpu_tss_rw) + (TSS_ist + (x) * 8)
-
 /**
  * idtentry_mce_db - Macro to generate entry stubs for #MC and #DB
  * @vector:		Vector number
@@ -416,10 +411,6 @@ SYM_CODE_END(\asmsym)
  * If hits in kernel mode then it needs to go through the paranoid
  * entry as the exception can hit any random state. No preemption
  * check on exit to keep the paranoid path simple.
- *
- * If the trap is #DB then the interrupt stack entry in the IST is
- * moved to the second stack, so a potential recursion will have a
- * fresh IST.
  */
 .macro idtentry_mce_db vector asmsym cfunc
 SYM_CODE_START(\asmsym)
@@ -445,16 +436,8 @@ SYM_CODE_START(\asmsym)
 
 	movq	%rsp, %rdi		/* pt_regs pointer */
 
-	.if \vector == X86_TRAP_DB
-		subq	$DB_STACK_OFFSET, CPU_TSS_IST(IST_INDEX_DB)
-	.endif
-
 	call	\cfunc
 
-	.if \vector == X86_TRAP_DB
-		addq	$DB_STACK_OFFSET, CPU_TSS_IST(IST_INDEX_DB)
-	.endif
-
 	jmp	paranoid_exit
 
 	/* Switch to the regular task stack and use the noist entry point */
diff --git a/arch/x86/kernel/asm-offsets_64.c b/arch/x86/kernel/asm-offsets_64.c
index c2a47016f243..472378330169 100644
--- a/arch/x86/kernel/asm-offsets_64.c
+++ b/arch/x86/kernel/asm-offsets_64.c
@@ -57,7 +57,6 @@ int main(void)
 	BLANK();
 #undef ENTRY
 
-	OFFSET(TSS_ist, tss_struct, x86_tss.ist);
 	DEFINE(DB_STACK_OFFSET, offsetof(struct cea_exception_stacks, DB_stack) -
 	       offsetof(struct cea_exception_stacks, DB1_stack));
 	BLANK();
-- 
2.20.1