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, 9 insertions, 8 deletions

diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 37230342c2c4..95ff6a0e942a 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -323,13 +323,14 @@ static void load_TLS_descriptor(struct thread_struct *t,
 static void xen_load_tls(struct thread_struct *t, unsigned int cpu)
 {
         /*
-         * XXX sleazy hack: If we're being called in a lazy-cpu zone,
-         * it means we're in a context switch, and %gs has just been
-         * saved.  This means we can zero it out to prevent faults on
-         * exit from the hypervisor if the next process has no %gs.
-         * Either way, it has been saved, and the new value will get
-         * loaded properly.  This will go away as soon as Xen has been
-         * modified to not save/restore %gs for normal hypercalls.
+         * XXX sleazy hack: If we're being called in a lazy-cpu zone
+         * and lazy gs handling is enabled, it means we're in a
+         * context switch, and %gs has just been saved.  This means we
+         * can zero it out to prevent faults on exit from the
+         * hypervisor if the next process has no %gs.  Either way, it
+         * has been saved, and the new value will get loaded properly.
+         * This will go away as soon as Xen has been modified to not
+         * save/restore %gs for normal hypercalls.
          *
          * On x86_64, this hack is not used for %gs, because gs points
          * to KERNEL_GS_BASE (and uses it for PDA references), so we
@@ -341,7 +342,7 @@ static void xen_load_tls(struct thread_struct *t, unsigned int cpu)
          */
         if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_CPU) {
 #ifdef CONFIG_X86_32
-                loadsegment(gs, 0);
+                lazy_load_gs(0);
 #else
                 loadsegment(fs, 0);
 #endif