s390: add support for virtually mapped kernel stacks

With virtually mapped kernel stacks the kernel stack overflow detection
is now fault based, every stack has a guard page in the vmalloc space.
The panic_stack is renamed to nodat_stack and is used for all function
that need to run without DAT, e.g. memcpy_real or do_start_kdump.

The main effect is a reduction in the kernel image size as with vmap
stacks the old style overflow checking that adds two instructions per
function is not needed anymore. Result from bloat-o-meter:

add/remove: 20/1 grow/shrink: 13/26854 up/down: 2198/-216240 (-214042)

In regard to performance the micro-benchmark for fork has a hit of a
few microseconds, allocating 4 pages in vmalloc space is more expensive
compare to an order-2 page allocation. But with real workload I could
not find a noticeable difference.

Acked-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/arch/s390/include/asm/processor.h b/arch/s390/include/asm/processor.h
index 43494a014d5b..3c1e723a143a 100644
--- a/arch/s390/include/asm/processor.h
+++ b/arch/s390/include/asm/processor.h
@@ -163,6 +163,14 @@ struct thread_struct {
 typedef struct thread_struct thread_struct;
 
 /*
+ * General size of a stack
+ */
+#define STACK_ORDER 2
+#define STACK_SIZE (PAGE_SIZE << STACK_ORDER)
+#define STACK_INIT_OFFSET \
+	(STACK_SIZE - STACK_FRAME_OVERHEAD - sizeof(struct pt_regs))
+
+/*
  * Stack layout of a C stack frame.
  */
 #ifndef __PACK_STACK