x86, ptrace: support for branch trace store(BTS)

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Changes to the last version:
- split implementation into two layers: ds/bts and ptrace
- renamed TIF's
- save/restore ds save area msr in __switch_to_xtra()
- make block-stepping only look at BTF bit

Signed-off-by: Markus Metzger <markus.t.metzger@intel.com>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 13 insertions, 5 deletions

diff --git a/arch/x86/kernel/step.c b/arch/x86/kernel/step.c
index f55c003..21ea22f 100644
--- a/arch/x86/kernel/step.c
+++ b/arch/x86/kernel/step.c
@@ -169,9 +169,14 @@ static void enable_step(struct task_struct *child, bool block)
 	 */
 	if (enable_single_step(child) && block) {
 		set_tsk_thread_flag(child, TIF_DEBUGCTLMSR);
-		write_debugctlmsr(child, DEBUGCTLMSR_BTF);
-	} else if (test_and_clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR)) {
-		write_debugctlmsr(child, 0);
+		write_debugctlmsr(child,
+				  child->thread.debugctlmsr | DEBUGCTLMSR_BTF);
+	} else {
+	    write_debugctlmsr(child,
+			      child->thread.debugctlmsr & ~TIF_DEBUGCTLMSR);
+
+	    if (!child->thread.debugctlmsr)
+		    clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR);
 	}
 }
 
@@ -190,8 +195,11 @@ void user_disable_single_step(struct task_struct *child)
 	/*
 	 * Make sure block stepping (BTF) is disabled.
 	 */
-	if (test_and_clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR))
-		write_debugctlmsr(child, 0);
+	write_debugctlmsr(child,
+			  child->thread.debugctlmsr & ~TIF_DEBUGCTLMSR);
+
+	if (!child->thread.debugctlmsr)
+		clear_tsk_thread_flag(child, TIF_DEBUGCTLMSR);
 
 	/* Always clear TIF_SINGLESTEP... */
 	clear_tsk_thread_flag(child, TIF_SINGLESTEP);