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, 13 insertions, 4 deletions

diff --git a/arch/s390/kernel/machine_kexec.c b/arch/s390/kernel/machine_kexec.c
index b7020e721ae3..cb582649aba6 100644
--- a/arch/s390/kernel/machine_kexec.c
+++ b/arch/s390/kernel/machine_kexec.c
@@ -142,18 +142,27 @@ static noinline void __machine_kdump(void *image)
 }
 #endif
 
+static unsigned long do_start_kdump(unsigned long addr)
+{
+	struct kimage *image = (struct kimage *) addr;
+	int (*start_kdump)(int) = (void *)image->start;
+	int rc;
+
+	__arch_local_irq_stnsm(0xfb); /* disable DAT */
+	rc = start_kdump(0);
+	__arch_local_irq_stosm(0x04); /* enable DAT */
+	return rc;
+}
+
 /*
  * Check if kdump checksums are valid: We call purgatory with parameter "0"
  */
 static bool kdump_csum_valid(struct kimage *image)
 {
 #ifdef CONFIG_CRASH_DUMP
-	int (*start_kdump)(int) = (void *)image->start;
 	int rc;
 
-	__arch_local_irq_stnsm(0xfb); /* disable DAT */
-	rc = start_kdump(0);
-	__arch_local_irq_stosm(0x04); /* enable DAT */
+	rc = CALL_ON_STACK(do_start_kdump, S390_lowcore.nodat_stack, 1, image);
 	return rc == 0;
 #else
 	return false;