s390/smp: enable DAT before CPU restart callback is called

The restart interrupt is triggered whenever a secondary CPU is
brought online, a remote function call dispatched from another
CPU or a manual PSW restart is initiated and causes the system
to kdump. The handling routine is always called with DAT turned
off. It then initializes the stack frame and invokes a callback.

The existing callbacks handle DAT as follows:

  * __do_restart() and __machine_kexec() turn in on upon entry;
  * __ipl_run(), __reipl_run() and __dump_run() do not turn it
    right away, but all of them call diag308() - which turns DAT
    on, but only if kasan is enabled;

In addition to the described complexity all callbacks (and the
functions they call) should avoid kasan instrumentation while
DAT is off.

This update enables DAT in the assembler restart handler and
relieves any callbacks (which are mostly C functions) from
dealing with DAT altogether.

There are four types of CPU restart that initialize control
registers in different ways:

  1. Start of secondary CPU on boot - control registers are
     inherited from the IPL CPU;
  2. Restart of online CPU - control registers of the CPU being
     restarted are kept;
  3. Hotplug of offline CPU - control registers are inherited
     from the starting CPU;
  4. Start of offline CPU triggered by manual PSW restart -
     the control registers are read from the absolute lowcore
     and contain the boot time IPL CPU values updated with all
     follow-up calls of smp_ctl_set_bit() and smp_ctl_clear_bit()
     routines;

In first three cases contents of the control registers is the
most recent. In the latter case control registers are good
enough to facilitate successful completion of kdump operation.

Suggested-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

1 files changed, 0 insertions, 3 deletions

diff --git a/arch/s390/kernel/ipl.c b/arch/s390/kernel/ipl.c
index 546d729292cf..e2cc35775b99 100644
--- a/arch/s390/kernel/ipl.c
+++ b/arch/s390/kernel/ipl.c
@@ -179,8 +179,6 @@ static inline int __diag308(unsigned long subcode, void *addr)
 
 int diag308(unsigned long subcode, void *addr)
 {
-	if (IS_ENABLED(CONFIG_KASAN))
-		__arch_local_irq_stosm(0x04); /* enable DAT */
 	diag_stat_inc(DIAG_STAT_X308);
 	return __diag308(subcode, addr);
 }
@@ -1843,7 +1841,6 @@ static struct kobj_attribute on_restart_attr = __ATTR_RW(on_restart);
 
 static void __do_restart(void *ignore)
 {
-	__arch_local_irq_stosm(0x04); /* enable DAT */
 	smp_send_stop();
 #ifdef CONFIG_CRASH_DUMP
 	crash_kexec(NULL);