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>
diff --git a/arch/x86/xen/enlighten.c b/arch/x86/xen/enlighten.c
index 3723034..95ff6a0 100644
--- a/arch/x86/xen/enlighten.c
+++ b/arch/x86/xen/enlighten.c
@@ -323,13 +323,14 @@
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 @@
*/
if (paravirt_get_lazy_mode() == PARAVIRT_LAZY_CPU) {
#ifdef CONFIG_X86_32
- loadsegment(gs, 0);
+ lazy_load_gs(0);
#else
loadsegment(fs, 0);
#endif