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, 13 insertions, 2 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)