x86: make lazy %gs optional on x86_32

Impact: pt_regs changed, lazy gs handling made optional, add slight
        overhead to SAVE_ALL, simplifies error_code path a bit

On x86_32, %gs hasn't been used by kernel and handled lazily.  pt_regs
doesn't have place for it and gs is saved/loaded only when necessary.
In preparation for stack protector support, this patch makes lazy %gs
handling optional by doing the followings.

* Add CONFIG_X86_32_LAZY_GS and place for gs in pt_regs.

* Save and restore %gs along with other registers in entry_32.S unless
  LAZY_GS.  Note that this unfortunately adds "pushl $0" on SAVE_ALL
  even when LAZY_GS.  However, it adds no overhead to common exit path
  and simplifies entry path with error code.

* Define different user_gs accessors depending on LAZY_GS and add
  lazy_save_gs() and lazy_load_gs() which are noop if !LAZY_GS.  The
  lazy_*_gs() ops are used to save, load and clear %gs lazily.

* Define ELF_CORE_COPY_KERNEL_REGS() which always read %gs directly.

xen and lguest changes need to be verified.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Jeremy Fitzhardinge <jeremy@xensource.com>
Cc: Rusty Russell <rusty@rustcorp.com.au>

Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 11 insertions, 1 deletions

diff --git a/arch/x86/include/asm/system.h b/arch/x86/include/asm/system.h
index 70c74b8db875..79b98e5b96f4 100644
--- a/arch/x86/include/asm/system.h
+++ b/arch/x86/include/asm/system.h
@@ -186,10 +186,20 @@ extern void native_load_gs_index(unsigned);
  * x86_32 user gs accessors.
  */
 #ifdef CONFIG_X86_32
+#ifdef CONFIG_X86_32_LAZY_GS
 #define get_user_gs(regs)       (u16)({unsigned long v; savesegment(gs, v); v;})
 #define set_user_gs(regs, v)    loadsegment(gs, (unsigned long)(v))
 #define task_user_gs(tsk)       ((tsk)->thread.gs)
-#endif
+#define lazy_save_gs(v)         savesegment(gs, (v))
+#define lazy_load_gs(v)         loadsegment(gs, (v))
+#else   /* X86_32_LAZY_GS */
+#define get_user_gs(regs)       (u16)((regs)->gs)
+#define set_user_gs(regs, v)    do { (regs)->gs = (v); } while (0)
+#define task_user_gs(tsk)       (task_pt_regs(tsk)->gs)
+#define lazy_save_gs(v)         do { } while (0)
+#define lazy_load_gs(v)         do { } while (0)
+#endif  /* X86_32_LAZY_GS */
+#endif  /* X86_32 */
 
 static inline unsigned long get_limit(unsigned long segment)
 {