amd64: Bump MAXCPU to 1024 (from 256)

Hardware with more than 256 CPU cores is currently available and will
become increasingly common over FreeBSD 14's lifetime.  Increase MAXCPU
in the amd64 GENERIC kernel configuration to 1024.

Earlier commits increased some related limits.  These prerequisite
commits include at least:

- d7ed40243769 Increase MAX_APIC_ID safeguard to 0x800
- d1639e43c589 cpuset: increase userland maximum size to 1024

Global and allocated arrays sized by MAXCPU result in excessive bloat
on systems with lower core counts.  In addition, some code used u_char
(8 bits) to hold a CPU index, which is not valid if MAXCPU is greater
than 256.

A number of recent commits addressed these sorts of issues, including
at least:

- 133935d26f20 pf: atomically increment state ids
- 74ac712f72cf vmm: Dynamically allocate a couple of per-CPU state save areas
- 78cfa762ebf2 callout: Move per-CPU callout state into the dpcpu region
- 42f722e721cd amd64: store pcids pmap data in pcpu zone
- 9801e7c275f6 smp_topo: dynamically allocate group array
- 9fb6718d1b18 smp: Dynamically allocate the stoppcbs array
- 2bb16c635249 x86: retire use of intr_bind

There are some additional allocations still to be converted and
more scalability work is required to make effective use of very high
core count systems, but this change allows us to boot on these systems
and provides a Kernel Binary Interface (KBI) for the FreeBSD 14 release
that supports these configurations.

Special thanks to AMD for providing hardware to test these changes.

PR:		269572
Reviewed by:	des
Relnotes:	Yes
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36838

2 files changed, 5 insertions, 1 deletions

diff --git a/UPDATING b/UPDATING
index 8e1b3e8555ba..5889c0ac9903 100644
--- a/UPDATING
+++ b/UPDATING
@@ -27,6 +27,10 @@ NOTE TO PEOPLE WHO THINK THAT FreeBSD 14.x IS SLOW:
 	world, or to merely disable the most expensive debugging functionality
 	at runtime, run "ln -s 'abort:false,junk:false' /etc/malloc.conf".)
 
+20230803:
+	MAXCPU has been increased to 1024 in the amd64 GENERIC kernel config.
+	Out-of-tree kernel modules will need to be rebuilt.
+
 20230724:
 	CAM has been mechanically updated s/u_int(64|32|16|8)_t/uint\1_t/g
 	to move to the standard uintXX_t types from the old, traditional
diff --git a/sys/amd64/include/param.h b/sys/amd64/include/param.h
index a76be23bbe91..c7e5a5c7fc42 100644
--- a/sys/amd64/include/param.h
+++ b/sys/amd64/include/param.h
@@ -65,7 +65,7 @@
 
 #ifdef SMP
 #ifndef MAXCPU
-#define MAXCPU		256
+#define MAXCPU		1024
 #endif
 #else
 #define MAXCPU		1