From defcbbc6527cd20bd45e42fab53d7e185156df48 Mon Sep 17 00:00:00 2001
From: "H. Peter Anvin" <hpa@zytor.com>
Date: Fri, 15 Feb 2008 22:35:18 -0800
Subject: [PATCH] Don't close the A20 gate; leave it open

Don't close the A20 gate (restore to previous state); leave it open.
This is good for performance, but also avoids a nasty race condition
when leaving protected mode to then quickly re-enter protected mode.
---
 bcopy32.inc |  9 ++++++++-
 com32.inc   |  2 ++
 config.inc  | 11 +++++++++++
 3 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/bcopy32.inc b/bcopy32.inc
index b62d3ac..99a16d8 100644
--- a/bcopy32.inc
+++ b/bcopy32.inc
@@ -190,7 +190,9 @@ simple_pm_call:
 		pop fs
 		pop es
 		pop ds
+%if DISABLE_A20
 		call disable_a20
+%endif
 
 		popfd			; Re-enables interrupts
 		pop ebp
@@ -412,6 +414,8 @@ a20_test:
 		pop es
 		ret
 
+%if DISABLE_A20
+
 disable_a20:
 		pushad
 ;
@@ -464,6 +468,8 @@ a20d_none:
 		popad
 		ret
 
+%endif
+
 ;
 ; Routine to empty the 8042 KBC controller.  If dl != 0
 ; then we will test A20 in the loop and exit if A20 is
@@ -594,8 +600,9 @@ trampoline_to_pm:
 
 		align 2
 A20List		dw a20_dunno, a20_none, a20_bios, a20_kbc, a20_fast
+%if DISABLE_A20
 A20DList	dw a20d_dunno, a20d_none, a20d_bios, a20d_kbc, a20d_fast
-a20_adjust_cnt	equ ($-A20List)/2
+%endif
 
 A20Type		dw A20_NONE		; A20 type
 
diff --git a/com32.inc b/com32.inc
index c90c9ec..220c9d7 100644
--- a/com32.inc
+++ b/com32.inc
@@ -210,7 +210,9 @@ com32_enter_rm:
 		jmp bx			; Go to whereever we need to go...
 
 com32_done:
+%if DISABLE_A20
 		call disable_a20
+%endif
 		sti
 		jmp enter_command
 
diff --git a/config.inc b/config.inc
index d199597..d6a981e 100644
--- a/config.inc
+++ b/config.inc
@@ -28,6 +28,17 @@ MAX_FKEYS	equ 12			; Number of F-key help files
 %assign	DO_WBINVD 0			; Should we use WBINVD or not?
 
 ;
+; Set this to return the A20 gate to its previous state, instead of
+; leaving it open.  This has caused problems, because there appear
+; to be a race condition between disabling the A20 gate and trying to
+; re-enter protected mode, causing the A20 gate disable to take effect
+; after we have already done the A20 enabled check, with disastrous
+; consequences.  Plus, there seems to be little or no demand for it.
+;
+%assign DISABLE_A20	0
+
+
+;
 ; Version number definitinons
 ;
 %ifndef DEPEND				; Generated file
-- 
2.7.4