From f849557c06e55e7ebedc1b5c873fdf5111587918 Mon Sep 17 00:00:00 2001
From: Mark Johnston <markj@FreeBSD.org>
Date: Tue, 7 Jan 2025 17:58:58 +0000
Subject: [PATCH] x86: Short-circuit ipi_all_but_self() on UP systems

Apparently this is required on old intel hw, see
https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=275086#c3

PR:		275086
Reviewed by:	mav, kib
Fixes:		279cd05b7e4d ("Use APIC_IPI_DEST_OTHERS for bitmapped IPIs too.")
MFC after:	1 week
Diagnosed by:	Ben Wilber <ben@desync.com>
Differential Revision:	https://reviews.freebsd.org/D48361

(cherry picked from commit 254a2b767f9a39f1541e0a07a70bbe269e86ad70)
---
 sys/x86/x86/mp_x86.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/sys/x86/x86/mp_x86.c b/sys/x86/x86/mp_x86.c
index 11b11471d73..c52102e2de1 100644
--- a/sys/x86/x86/mp_x86.c
+++ b/sys/x86/x86/mp_x86.c
@@ -1427,6 +1427,9 @@ ipi_all_but_self(u_int ipi)
 	cpuset_t other_cpus;
 	int cpu, c;
 
+	if (mp_ncpus == 1)
+		return;
+
 	/*
 	 * IPI_STOP_HARD maps to a NMI and the trap handler needs a bit
 	 * of help in order to understand what is the source.