git: f8a199f28f9d - main - stand: Raise limit to 550,000 bytes for loader

The branch main has been updated by imp:

URL: https://cgit.FreeBSD.org/src/commit/?id=f8a199f28f9de46fe7360c3a4a0e19e71be72721

commit f8a199f28f9de46fe7360c3a4a0e19e71be72721
Author:     Warner Losh <imp@FreeBSD.org>
AuthorDate: 2022-08-12 04:59:51 +0000
Commit:     Warner Losh <imp@FreeBSD.org>
CommitDate: 2022-08-13 03:47:02 +0000

    stand: Raise limit to 550,000 bytes for loader
    
    Raise the limit for /boot/loader to be 550k. The IBM PC imposes a limit
    of 640k of RAM below 1MB, which is needed for real mode calls. BTX takes
    40k of that. The BIOS takes some amount (25k seems a good "99% take less
    than or equal to this" estimate for that, though some systems consume
    more). Most typical setups need 25k of stack.  This leaves 550k for
    code. We set the limit to 550,000 which gives about an extra 13,000
    bytes of buffer for machines that whose setups use a little more stack
    or whose BIOS reserves a bit more...
    
    Add this derivation in the Makefile. Also recommend setting LOADERSIZE
    lower in /etc/src.conf when the loader has to run on a system whose BIOS
    takes up more space, or for a complex setup. Add a recipe for how to
    find how much RAM your BIOS uses as well (thanks to jhb@ for the
    trick). Network cards that boot via PXE and HBAs with their BIOS enabled
    are known to be large consumers of lomem space.
    
    Sponsored by:           Netflix
    Reviewed by:            jhb
    Differential Revision:  https://reviews.freebsd.org/D36152
---
 stand/i386/loader/Makefile | 18 ++++++++++++++++--
 1 file changed, 16 insertions(+), 2 deletions(-)

diff --git a/stand/i386/loader/Makefile b/stand/i386/loader/Makefile
index 539c736af38f..723220484e67 100644
--- a/stand/i386/loader/Makefile
+++ b/stand/i386/loader/Makefile
@@ -19,8 +19,22 @@ PROG=		${LOADER}.sym
 INTERNALPROG=
 NEWVERSWHAT?=	"bootstrap loader" x86
 VERSION_FILE=	${.CURDIR}/../loader/version
-LOADERSIZE?=	508000		# Largest known safe size for loader.bin
-
+#
+# There's 640k - 40k maximum space, less however much memory the BIOS uses.  A
+# non-random survey suggests that 20k-25k is a good value for 'most' machines.
+# We also need to subtract maximum stack usage (20-25k).
+#
+# So 640k - 40k - 25k - 25k = 550k = 563,200 bytes, but use 550,000 below for
+# some extra buffer for more complex setups and/or wider BIOS lomem variation.
+#
+# Some systems use more stack or have BIOS resere more RAM (or both), and will
+# need to set this lower in /etc/loader.conf if the above buffer is too small.
+#
+# sudo dd if=/dev/mem bs=1 iseek=0x413 count=2 | hd -d
+#
+# will tell you how many kiB of lomem are available.
+#
+LOADERSIZE?=	550000		# Largest known safe size for loader.bin
 
 .PATH:		${BOOTSRC}/i386/loader