x86, ptrace: Fix block-step

Implement ptrace-block-step using TIF_BLOCKSTEP which will set
DEBUGCTLMSR_BTF when set for a task while preserving any other
DEBUGCTLMSR bits.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
LKML-Reference: <20100325135414.017536066@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 11 insertions, 0 deletions

diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
index 1a60beb32ede..8328009416d7 100644
--- a/arch/x86/kernel/process.c
+++ b/arch/x86/kernel/process.c
@@ -195,6 +195,17 @@ void __switch_to_xtra(struct task_struct *prev_p, struct task_struct *next_p,
         prev = &prev_p->thread;
         next = &next_p->thread;
 
+        if (test_tsk_thread_flag(prev_p, TIF_BLOCKSTEP) ^
+            test_tsk_thread_flag(next_p, TIF_BLOCKSTEP)) {
+                unsigned long debugctl = get_debugctlmsr();
+
+                debugctl &= ~DEBUGCTLMSR_BTF;
+                if (test_tsk_thread_flag(next_p, TIF_BLOCKSTEP))
+                        debugctl |= DEBUGCTLMSR_BTF;
+
+                update_debugctlmsr(debugctl);
+        }
+
         if (test_tsk_thread_flag(prev_p, TIF_NOTSC) ^
             test_tsk_thread_flag(next_p, TIF_NOTSC)) {
                 /* prev and next are different */