powerpc/pseries, ps3: panic flush kernel messages before halting system

Platforms with a panic handler that halts the system can have problems
getting kernel messages out, because the panic notifiers are called
before kernel/panic.c does its flushing of printk buffers an console
etc.

This was attempted to be solved with commit a3b2cb30f2 ("powerpc: Do
not call ppc_md.panic in fadump panic notifier"), but that wasn't the
right approach and caused other problems, and was reverted by commit
ab9dbf771f.

Instead, the powernv shutdown paths have already had a similar
problem, fixed by taking the message flushing sequence from
kernel/panic.c. That's a little bit ugly, but while we have the code
duplicated, it will work for this case as well. So have ppc panic
handlers do the same flushing before they terminate.

Without this patch, a qemu pseries_le_defconfig guest stops silently
when issued the nmi command when xmon is off and no crash dumpers
enabled. Afterwards, an oops is printed by each CPU as expected.

Fixes: ab9dbf771f ("Revert "powerpc: Do not call ppc_md.panic in fadump panic notifier"")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/platforms/powernv/opal.c

@@ -461,24 +461,14 @@ static int opal_recover_mce(struct pt_regs *regs,
 
 void pnv_platform_error_reboot(struct pt_regs *regs, const char *msg)
 {
-	/*
-	 * This is mostly taken from kernel/panic.c, but tries to do
-	 * relatively minimal work. Don't use delay functions (TB may
-	 * be broken), don't crash dump (need to set a firmware log),
-	 * don't run notifiers. We do want to get some information to
-	 * Linux console.
-	 */
-	console_verbose();
-	bust_spinlocks(1);
+	panic_flush_kmsg_start();
+
 	pr_emerg("Hardware platform error: %s\n", msg);
 	if (regs)
 		show_regs(regs);
 	smp_send_stop();
-	printk_safe_flush_on_panic();
-	kmsg_dump(KMSG_DUMP_PANIC);
-	bust_spinlocks(0);
-	debug_locks_off();
-	console_flush_on_panic();
+
+	panic_flush_kmsg_end();
 
 	/*
 	 * Don't bother to shut things down because this will