stand: Reduce limit to 500k for x86 loader

The largest loader that works for PXE boot is about 500k. PXE needs low
memory for packets and other driver state, so the largest safe size for
the loader is about 500k. Reduce the size from 560k to 500k so we don't
accidentally break PXE in the future.

Add a comment for people with special needs. If you control the
hardware, it can be safe to have boot loaders as large as 580k or 600k
in some cases. Since the BIOS loader is becoming more and more of a
legacy item, the build variable LOADERSIZE isn't documented. This change
doesn't change that: there's been little demand for this documentation
and in general, users shouldn't change it lightly.

PR: 257018
Sponsored by: Netflix

1 files changed, 6 insertions, 1 deletions

diff --git a/stand/i386/loader/Makefile b/stand/i386/loader/Makefile
index a4aa3a3c4d45..efd442977780 100644
--- a/stand/i386/loader/Makefile
+++ b/stand/i386/loader/Makefile
@@ -32,7 +32,12 @@ VERSION_FILE=	${.CURDIR}/../loader/version
 #
 # will tell you how many kiB of lomem are available.
 #
-LOADERSIZE?=	560000		# Largest known safe size for loader.bin
+# We further reduce this to 500k, though, to give PXE an additional 64k of space
+# so pxeloader will fit. If you have special needs that do not include pxeboot,
+# you can safely set this as high as 560000 generally, or a bit higher if you
+# have tight control over the machines you are booting on.
+#
+LOADERSIZE?=	500000		# Largest known safe size for loader.bin
 
 .PATH:		${BOOTSRC}/i386/loader