[RFC][PATCH 1/4] x86/entry: Introduce local_db_{rave,restore}()

Peter Zijlstra
2020-05-22T20:50:11Z

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

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
arch/x86/include/asm/debugreg.h | 23 +++++++++++++++++++++++
arch/x86/kernel/traps.c | 18 ++----------------
2 files changed, 25 insertions(+), 16 deletions(-)

--- a/arch/x86/include/asm/debugreg.h
+++ b/arch/x86/include/asm/debugreg.h
@@ -113,6 +113,29 @@ static inline void debug_stack_usage_inc
static inline void debug_stack_usage_dec(void) { }
#endif /* X86_64 */

+static __always_inline void local_db_save(unsigned long *dr7)
+{
+ 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();
+}
+
+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();
+ set_debugreg(dr7, 7);
+}
+
#ifdef CONFIG_CPU_SUP_AMD
extern void set_dr_addr_mask(unsigned long mask, int dr);
#else
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -727,15 +727,7 @@ static __always_inline void debug_enter(
* 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();
+ local_db_save(dr7);

/*
* The Intel SDM says:
@@ -756,13 +748,7 @@ static __always_inline void debug_enter(

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);
}

/*