linux-kernel - [RFC][PATCH 6/6] x86/entry: Remove DBn stacks

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Message-ID: <20200528202328.708732152@infradead.org>
Date:   Thu, 28 May 2020 22:19:43 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     tglx@...utronix.de, luto@...capital.net, peterz@...radead.org
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        sean.j.christopherson@...el.com, andrew.cooper3@...rix.com,
        daniel.thompson@...aro.org
Subject: [RFC][PATCH 6/6] x86/entry: Remove DBn stacks

Both #DB itself, as all other IST users (NMI, #MC) now clear DR7 on
entry. Combined with not allowing breakpoints on entry/noinstr/NOKPROBE
text and no single step (EFLAGS.TF) inside the #DB handler should
guarantee us no nested #DB.

XXX depends on KGDB breakpoints not being stupid

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 arch/x86/entry/entry_64.S             |   13 -------------
 arch/x86/include/asm/cpu_entry_area.h |    6 ------
 arch/x86/kernel/asm-offsets_64.c      |    3 ---
 arch/x86/kernel/dumpstack_64.c        |    3 ---
 arch/x86/mm/cpu_entry_area.c          |    1 -
 5 files changed, 26 deletions(-)

--- 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
@@ -445,16 +440,8 @@ 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 */
--- a/arch/x86/include/asm/cpu_entry_area.h
+++ b/arch/x86/include/asm/cpu_entry_area.h
@@ -16,10 +16,6 @@
 	char	DF_stack[EXCEPTION_STKSZ];	\
 	char	NMI_stack_guard[guardsize];	\
 	char	NMI_stack[EXCEPTION_STKSZ];	\
-	char	DB2_stack_guard[guardsize];	\
-	char	DB2_stack[db2_holesize];	\
-	char	DB1_stack_guard[guardsize];	\
-	char	DB1_stack[EXCEPTION_STKSZ];	\
 	char	DB_stack_guard[guardsize];	\
 	char	DB_stack[EXCEPTION_STKSZ];	\
 	char	MCE_stack_guard[guardsize];	\
@@ -42,8 +38,6 @@ struct cea_exception_stacks {
 enum exception_stack_ordering {
 	ESTACK_DF,
 	ESTACK_NMI,
-	ESTACK_DB2,
-	ESTACK_DB1,
 	ESTACK_DB,
 	ESTACK_MCE,
 	N_EXCEPTION_STACKS
--- a/arch/x86/kernel/asm-offsets_64.c
+++ b/arch/x86/kernel/asm-offsets_64.c
@@ -57,9 +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();
 
 #ifdef CONFIG_STACKPROTECTOR
--- a/arch/x86/kernel/dumpstack_64.c
+++ b/arch/x86/kernel/dumpstack_64.c
@@ -22,8 +22,6 @@
 static const char * const exception_stack_names[] = {
 		[ ESTACK_DF	]	= "#DF",
 		[ ESTACK_NMI	]	= "NMI",
-		[ ESTACK_DB2	]	= "#DB2",
-		[ ESTACK_DB1	]	= "#DB1",
 		[ ESTACK_DB	]	= "#DB",
 		[ ESTACK_MCE	]	= "#MC",
 };
@@ -79,7 +77,6 @@ static const
 struct estack_pages estack_pages[CEA_ESTACK_PAGES] ____cacheline_aligned = {
 	EPAGERANGE(DF),
 	EPAGERANGE(NMI),
-	EPAGERANGE(DB1),
 	EPAGERANGE(DB),
 	EPAGERANGE(MCE),
 };
--- a/arch/x86/mm/cpu_entry_area.c
+++ b/arch/x86/mm/cpu_entry_area.c
@@ -107,7 +107,6 @@ static void __init percpu_setup_exceptio
 	 */
 	cea_map_stack(DF);
 	cea_map_stack(NMI);
-	cea_map_stack(DB1);
 	cea_map_stack(DB);
 	cea_map_stack(MCE);
 }