kernel: introduce per-CPU panic(9) message buffers

Add a 512-byte buffer (ci_panicbuf) to each cpu_info struct on each
platform for use by panic(9).  The first panic on a given CPU writes
its message to this buffer.  Subsequent panics on a given CPU print
the panic message to the console but do not modify the buffer.  This
aids debugging in two cases:

- If 2+ CPUs panic simultaneously there is no risk of garbled messages
  in the panic buffer.

- If a CPU panics and then the operator causes a second panic while
  using ddb(4), the operator can still recall the first failure on
  a particular CPU.

Misc. changes to support this bigger change:

- Set panicstr atomically to identify the first CPU to reach panic().

- Tweak db_show_panic_cmd() to print all panic messages across all
  CPUs.  Prefix the first panic with an asterisk ('*').

- Prefer db_printf() to printf() during a panic if we have it.
  Apparently it disturbs less global state.

- On amd64, tweak fault() to write the local panic buffer.  This needs
  more work.

Prompted by bluhm@ and deraadt@.  Mostly written by deraadt@.
Discussed with bluhm@, deraadt@ and kettenis@.

Borne from a discussion on tech@ about making panic(9) more MP-safe:

https://marc.info/?l=openbsd-tech&m=162086462316143&w=2

ok kettenis@, visa@, bluhm@, deraadt@

1 files changed, 3 insertions, 1 deletions

diff --git a/sys/arch/arm/include/cpu.h b/sys/arch/arm/include/cpu.h
index 118381e7fc7..28f1c9186ee 100644
--- a/sys/arch/arm/include/cpu.h
+++ b/sys/arch/arm/include/cpu.h
@@ -1,4 +1,4 @@
-/*	$OpenBSD: cpu.h,v 1.59 2020/05/31 06:23:57 dlg Exp $	*/
+/*	$OpenBSD: cpu.h,v 1.60 2021/06/02 00:39:26 cheloha Exp $	*/
 /*	$NetBSD: cpu.h,v 1.34 2003/06/23 11:01:08 martin Exp $	*/
 
 /*
@@ -198,6 +198,8 @@ struct cpu_info {
 #ifdef GPROF
 	struct gmonparam *ci_gmon;
 #endif
+
+	char			ci_panicbuf[512];
 };
 
 #define CPUF_PRIMARY 		(1<<0)