From 982ce17927a31626b24828726ba23b9fc462a95b Mon Sep 17 00:00:00 2001
From: Vladimir Serbinenko <phcoder@gmail.com>
Date: Sat, 22 Jul 2023 16:51:44 +0200
Subject: [PATCH] Prekernel: Map entire 4GiB space

Prekernel code currently assumes that mapping until MAX_KERNEL_SIZE
is enough to make the modules accessible. GRUB tries to load as low
as possible but higher than 1 MiB. Hence this is usually true.
However on EFI some ranges may already be used by boot services and
GRUB tries to avoid them if possible. This pushes modules higher.
The simplest solution is to map entire 4 GiB space.
As an additional benefit it makes the framebuffer accessible that
can be used for the debugging.
---
 Kernel/Prekernel/boot.S | 30 +++++++++++++++++++++++-------
 1 file changed, 23 insertions(+), 7 deletions(-)

diff --git a/Kernel/Prekernel/boot.S b/Kernel/Prekernel/boot.S
index a5fa487a5f..75eed5c1d3 100644
--- a/Kernel/Prekernel/boot.S
+++ b/Kernel/Prekernel/boot.S
@@ -24,7 +24,7 @@ boot_pdpt:
 .skip 4096
 .global boot_pd0
 boot_pd0:
-.skip 4096
+.skip (4096 * 4)
 .global boot_pd0_pts
 boot_pd0_pts:
 .skip 4096 * (MAX_KERNEL_SIZE >> 21)
@@ -65,10 +65,10 @@ pml4t:
 
 pdpt
 
-    0: boot_pd0 (0-1GB)
-    1: n/a      (1-2GB)
-    2: n/a      (2-3GB)
-    3: n/a      (3-4GB)
+    0: boot_pd0             (0-1GB)
+    1: boot_pd0 + 4096      (1-2GB)
+    2: boot_pd0 + 4096 * 2  (2-3GB)
+    3: boot_pd0 + 4096 * 3  (3-4GB)
 
 boot_pd0 : 512 PDEs
 
@@ -421,13 +421,16 @@ long_mode_supported:
     xorl %eax, %eax
     rep stosl
 
-    /* set up pdpt[0] and pdpt[3] */
+    /* set up pdpt[0]..pdpt[3] */
     movl $boot_pdpt, %edi
     movl $(boot_pd0 + 3), 0(%edi)
+    movl $(boot_pd0 + 4096 + 3), 8(%edi)
+    movl $(boot_pd0 + 4096 * 2 + 3), 16(%edi)
+    movl $(boot_pd0 + 4096 * 3 + 3), 24(%edi)
 
     /* clear pd0 */
     movl $boot_pd0, %edi
-    movl $1024, %ecx
+    movl $4096, %ecx
     xorl %eax, %eax
     rep stosl
 
@@ -463,6 +466,19 @@ long_mode_supported:
     addl $4096, %eax
     loop 1b
 
+    /* Map the rest with 2MiB pages */
+    /* add boot_pd0_pts to boot_pd0 */
+    movl $(2048 - (MAX_KERNEL_SIZE >> 21)), %ecx
+    movl $(boot_pd0 + (MAX_KERNEL_SIZE >> 21) * 8), %edi
+    /* R/W + Present + 2 MiB */
+    movl $(MAX_KERNEL_SIZE | 0x83), %eax
+
+1:
+    movl %eax, 0(%edi)
+    addl $8, %edi
+    addl $(1 << 21), %eax
+    loop 1b
+
     /* point CR3 to PML4T */
     movl $boot_pml4t, %eax