From 1545a9a790c9882bb8e3e28e5fec858388149875 Mon Sep 17 00:00:00 2001
From: Lucia Ceionia <lucia@noreply.ceionius.com>
Date: Sat, 17 Sep 2022 16:31:41 -0500
Subject: [PATCH] V86 returns to caller

---
 Makefile     | 1 -
 handler.nasm | 5 ++---
 interrupt.c  | 3 ++-
 kernel.c     | 4 +++-
 v86.nasm     | 1 +
 5 files changed, 8 insertions(+), 6 deletions(-)

diff --git a/Makefile b/Makefile
index ca4eeb9..e8822d8 100644
--- a/Makefile
+++ b/Makefile
@@ -9,7 +9,6 @@ CFLAGS = -target "i686-elf" -m32 -ffreestanding -march=pentium-m -fno-stack-prot
 
 all: $(objects)
 	nasm boot.nasm -o boot.bin
-	# not sure why but if interrupt.c has any optimization everything just breaks immediately
 	gcc -Tlink.ld -m32 -ffreestanding -nostartfiles -nostdlib -o kernel.bin\
 		$(objects)
 	dd bs=256 count=1 conv=notrunc if=boot.bin of=virtdisk.bin
diff --git a/handler.nasm b/handler.nasm
index 34e1fa0..6320680 100644
--- a/handler.nasm
+++ b/handler.nasm
@@ -20,9 +20,8 @@ and eax, 1 << 17 ; VM flag
 test eax, eax
 pop eax
 jnz gpf_handler_v86
-mov word [0xb8000], 0x0f00 | 'G'
-mov word [0xb8002], 0x0f00 | 'P'
-mov word [0xb8004], 0x0f00 | 'F'
+mov dword [0xb8000], 0x0f000f00 | 'G' | 'P' << 16
+mov dword [0xb8004], 0x0f000f00 | 'F' | '!' << 16
 .hlt:
 hlt
 jmp .hlt
diff --git a/interrupt.c b/interrupt.c
index b4fea36..394221f 100644
--- a/interrupt.c
+++ b/interrupt.c
@@ -128,6 +128,7 @@ void gpf_handler_v86(struct interrupt_frame *frame, unsigned long error_code) {
     //    vga += (sizeof(uint8_t)*2)*2;
     //}
     vga = (char*)0xb8000 + (160*3);
+    uint32_t *tss_esp0 = (uint32_t*)0x20004;
     for(;;) {
         switch (ip[0]) {
             case 0x66: // O32
@@ -176,7 +177,7 @@ void gpf_handler_v86(struct interrupt_frame *frame, unsigned long error_code) {
                 vga[0] = 'I'; vga[2]++; if (vga[2] < '0') vga[2] = '0';
                 switch (ip[1]) {
                     case 0x30:
-                        asm ("jmp jmp_usermode_test");
+                        asm ("mov %%eax, %%esp\nret"::"a"(*tss_esp0));
                         for(;;);
                     case 0x3:
                         kbd_wait();
diff --git a/kernel.c b/kernel.c
index 01ba120..930ada7 100644
--- a/kernel.c
+++ b/kernel.c
@@ -52,8 +52,10 @@ void print_cr4() {
     printDword(reg, 0xB8000 + (160*5) + 50 + 8*4 + 4);
 }
 
+__attribute((__no_caller_saved_registers__))
 extern void enter_v86(uint32_t ss, uint32_t esp, uint32_t cs, uint32_t eip);
 extern void v86Code();
+__attribute((__no_caller_saved_registers__))
 extern void jmp_usermode_test();
 
 /*
@@ -107,6 +109,6 @@ void start() {
     print_cr4();
     FARPTR v86_entry = i386LinearToFp(v86Code);
     enter_v86(0x8000, 0xFF00, FP_SEG(v86_entry), FP_OFF(v86_entry));
-    //jmp_usermode_test();
+    jmp_usermode_test();
 }
 
diff --git a/v86.nasm b/v86.nasm
index cea6508..1ce84fe 100644
--- a/v86.nasm
+++ b/v86.nasm
@@ -71,6 +71,7 @@ jmp $
 global enter_v86
 enter_v86:
 mov ebp, esp               ; save stack pointer
+mov dword [0x20004], ebp   ; tss ESP0
 push dword  [ebp+4]        ; ss
 push dword  [ebp+8]        ; esp
 pushfd                     ; eflags