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>

4 files changed, 27 insertions, 6 deletions

diff --git a/arch/x86/include/asm/elf.h b/arch/x86/include/asm/elf.h
index 39b0aac1675c..83c1bc8d2e8a 100644
--- a/arch/x86/include/asm/elf.h
+++ b/arch/x86/include/asm/elf.h
@@ -112,7 +112,7 @@ extern unsigned int vdso_enabled;
  * now struct_user_regs, they are different)
  */
 
-#define ELF_CORE_COPY_REGS(pr_reg, regs)        \
+#define ELF_CORE_COPY_REGS_COMMON(pr_reg, regs) \
 do {                                            \
         pr_reg[0] = regs->bx;                   \
         pr_reg[1] = regs->cx;                   \
@@ -124,7 +124,6 @@ do {						\
         pr_reg[7] = regs->ds & 0xffff;          \
         pr_reg[8] = regs->es & 0xffff;          \
         pr_reg[9] = regs->fs & 0xffff;          \
-        pr_reg[10] = get_user_gs(regs);         \
         pr_reg[11] = regs->orig_ax;             \
         pr_reg[12] = regs->ip;                  \
         pr_reg[13] = regs->cs & 0xffff;         \
@@ -133,6 +132,18 @@ do {						\
         pr_reg[16] = regs->ss & 0xffff;         \
 } while (0);
 
+#define ELF_CORE_COPY_REGS(pr_reg, regs)        \
+do {                                            \
+        ELF_CORE_COPY_REGS_COMMON(pr_reg, regs);\
+        pr_reg[10] = get_user_gs(regs);         \
+} while (0);
+
+#define ELF_CORE_COPY_KERNEL_REGS(pr_reg, regs) \
+do {                                            \
+        ELF_CORE_COPY_REGS_COMMON(pr_reg, regs);\
+        savesegment(gs, pr_reg[10]);            \
+} while (0);
+
 #define ELF_PLATFORM    (utsname()->machine)
 #define set_personality_64bit() do { } while (0)
 
diff --git a/arch/x86/include/asm/mmu_context.h b/arch/x86/include/asm/mmu_context.h
index 4955165682c5..f923203dc39a 100644
--- a/arch/x86/include/asm/mmu_context.h
+++ b/arch/x86/include/asm/mmu_context.h
@@ -79,7 +79,7 @@ do {						\
 #ifdef CONFIG_X86_32
 #define deactivate_mm(tsk, mm)                  \
 do {                                            \
-        set_user_gs(task_pt_regs(tsk), 0);      \
+        lazy_load_gs(0);                        \
 } while (0)
 #else
 #define deactivate_mm(tsk, mm)                  \
diff --git a/arch/x86/include/asm/ptrace.h b/arch/x86/include/asm/ptrace.h
index 6d34d954c228..e304b66abeea 100644
--- a/arch/x86/include/asm/ptrace.h
+++ b/arch/x86/include/asm/ptrace.h
@@ -28,7 +28,7 @@ struct pt_regs {
         int  xds;
         int  xes;
         int  xfs;
-        /* int  gs; */
+        int  xgs;
         long orig_eax;
         long eip;
         int  xcs;
@@ -50,7 +50,7 @@ struct pt_regs {
         unsigned long ds;
         unsigned long es;
         unsigned long fs;
-        /* int  gs; */
+        unsigned long gs;
         unsigned long orig_ax;
         unsigned long ip;
         unsigned long cs;
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)
 {