[tip: x86/entry] x86/entry: Introduce local_db_{save,restore}()

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the x86/entry branch of tip:

Commit-ID:     57234891b3287b986e003876f906d95c9871e62e
Gitweb:        https://git.kernel.org/tip/57234891b3287b986e003876f906d95c9871e62e
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Fri, 29 May 2020 23:27:33 +02:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Sat, 30 May 2020 10:00:07 +02:00

x86/entry: Introduce local_db_{save,restore}()

In order to allow other exceptions than #DB to disable breakpoints,
provide common helpers.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/20200529213321.012060983@xxxxxxxxxxxxx

---
 arch/x86/include/asm/debugreg.h | 30 ++++++++++++++++++++++++++++++
 arch/x86/kernel/traps.c         | 18 ++----------------
 2 files changed, 32 insertions(+), 16 deletions(-)

diff --git a/arch/x86/include/asm/debugreg.h b/arch/x86/include/asm/debugreg.h
index 1a8609a..4ef8690 100644
--- a/arch/x86/include/asm/debugreg.h
+++ b/arch/x86/include/asm/debugreg.h
@@ -113,6 +113,36 @@ static inline void debug_stack_usage_inc(void) { }
 static inline void debug_stack_usage_dec(void) { }
 #endif /* X86_64 */
 
+static __always_inline unsigned long local_db_save(void)
+{
+	unsigned long dr7;
+
+	get_debugreg(dr7, 7);
+	dr7 &= ~0x400; /* architecturally set bit */
+	if (dr7)
+		set_debugreg(0, 7);
+	/*
+	 * Ensure the compiler doesn't lower the above statements into
+	 * the critical section; disabling breakpoints late would not
+	 * be good.
+	 */
+	barrier();
+
+	return dr7;
+}
+
+static __always_inline void local_db_restore(unsigned long dr7)
+{
+	/*
+	 * Ensure the compiler doesn't raise this statement into
+	 * the critical section; enabling breakpoints early would
+	 * not be good.
+	 */
+	barrier();
+	if (dr7)
+		set_debugreg(dr7, 7);
+}
+
 #ifdef CONFIG_CPU_SUP_AMD
 extern void set_dr_addr_mask(unsigned long mask, int dr);
 #else
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 50fb9cd..bcb9dd9 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -727,15 +727,7 @@ static __always_inline void debug_enter(unsigned long *dr6, unsigned long *dr7)
 	 * Entry text is excluded for HW_BP_X and cpu_entry_area, which
 	 * includes the entry stack is excluded for everything.
 	 */
-	get_debugreg(*dr7, 7);
-	set_debugreg(0, 7);
-
-	/*
-	 * Ensure the compiler doesn't lower the above statements into
-	 * the critical section; disabling breakpoints late would not
-	 * be good.
-	 */
-	barrier();
+	*dr7 = local_db_save();
 
 	/*
 	 * The Intel SDM says:
@@ -756,13 +748,7 @@ static __always_inline void debug_enter(unsigned long *dr6, unsigned long *dr7)
 
 static __always_inline void debug_exit(unsigned long dr7)
 {
-	/*
-	 * Ensure the compiler doesn't raise this statement into
-	 * the critical section; enabling breakpoints early would
-	 * not be good.
-	 */
-	barrier();
-	set_debugreg(dr7, 7);
+	local_db_restore(dr7);
 }
 
 /*