[PATCH] x86-64 TIF flags for debug regs and io bitmap in ctxsw

Hello,

Following my discussion with Andi. Here is a patch that introduces
two new TIF flags to simplify the context switch code in __switch_to().
The idea is to minimize the number of cache lines accessed in the common
case, i.e., when neither the debug registers nor the I/O bitmap are used.

This patch covers the x86-64 modifications. A patch for i386 follows.

Changelog:
	- add TIF_DEBUG to track when debug registers are active
	- add TIF_IO_BITMAP to track when I/O bitmap is used
	- modify __switch_to() to use the new TIF flags

<signed-off-by>: eranian@hpl.hp.com

Signed-off-by: Andi Kleen <ak@suse.de>

1 files changed, 6 insertions, 2 deletions

diff --git a/arch/x86_64/kernel/ptrace.c b/arch/x86_64/kernel/ptrace.c
index 2d50024c9f30..d35ec1bc696a 100644
--- a/arch/x86_64/kernel/ptrace.c
+++ b/arch/x86_64/kernel/ptrace.c
@@ -420,9 +420,13 @@ long arch_ptrace(struct task_struct *child, long request, long addr, long data)
                                 if ((0x5554 >> ((data >> (16 + 4*i)) & 0xf)) & 1)
                                         break;
                         if (i == 4) {
-                                child->thread.debugreg7 = data;
+                          child->thread.debugreg7 = data;
+                          if (data)
+                                set_tsk_thread_flag(child, TIF_DEBUG);
+                          else
+                                clear_tsk_thread_flag(child, TIF_DEBUG);
                           ret = 0;
-                  }
+                        }
                   break;
                 }
                 break;