[Qemu-devel] [PATCH 8/9] Multiboot -kernel support

[Alexander Graf]

To boot Mac OS X a mach bootloader has to be run. David Elliot modified
the original i386 Mac OS X BIOS based bootloader to emulate the EFI
entries and support multiboot, an easy but straight-forward kernel
loading interface founded by grub. To boot multiboot-compliant kernels
directly, this adds support for multiboot loading to the -kernel option,
if a Linux kernel was not found.

Index: qemu-snapshot-2008-01-08_05/hw/pc.c
===================================================================
--- qemu-snapshot-2008-01-08_05.orig/hw/pc.c
+++ qemu-snapshot-2008-01-08_05/hw/pc.c
@@ -493,6 +493,226 @@ static long get_file_size(FILE *f)
     return size;
 }
 
+/* Generate an initial boot sector which sets state and jump to
+   a specified vector */
+static void generate_bootsect_multiboot(uint32_t mh_entry_addr, uint32_t 
bootinfo)
+{
+    uint8_t bootsect[512], *p, *pgdt;
+    uint32_t ip;
+    int i;
+    int hda;
+
+    hda = drive_get_index(IF_IDE, 0, 0);
+    if (hda == -1) {
+       fprintf(stderr, "A disk image must be given for 'hda' when booting "
+               "a Multiboot kernel\n");
+       exit(1);
+        /* Copy the MSDOS partition table if possible */
+    }
+
+    memset(bootsect, 0, sizeof(bootsect));
+    bdrv_read(drives_table[hda].bdrv, 0, bootsect, 1);
+
+    /* Make sure we have a partition signature */
+    bootsect[510] = 0x55;
+    bootsect[511] = 0xaa;
+
+    /* Actual code */
+    p = bootsect;
+    *p++ = 0xfa;                /* CLI */
+    *p++ = 0xfc;                /* CLD */
+
+    // 660f011528000000               lgdt        [0x28]
+    *p++ = 0x66;                /* 32-bit operand size */
+    *p++ = 0x67;                /* 32-bit addr size */
+    *p++ = 0x0f;                /* LGDT [0x128] */
+    *p++ = 0x01;
+    *p++ = 0x15;
+    pgdt=p; /* we calculate the gdt position later */
+    p+=4;
+    
+    /* get us to protected mode now */
+    
+    *p++ = 0x66;
+    *p++ = 0xb8;                /* MOV EAX,0x01 */
+    *p++ = 0x01;
+    *p++ = 0x00;
+    *p++ = 0x00;
+    *p++ = 0x00;
+    
+    *p++ = 0x0f;                /* MOV CR0,EAX */
+    *p++ = 0x22;
+    *p++ = 0xc0;
+
+    /* the JMP sets CS for us and gets us to 32-bit */
+    ip = 0x00007c00 + (p - bootsect) + 8; // set i to the IP after the JMP
+    *p++ = 0x66;                /* 32-bit operand size */
+    *p++ = 0xea;                /* JMP */
+    *p++ = ip;        /* IP */
+    *p++ = ip >> 8;
+    *p++ = ip >> 16;
+    *p++ = ip >> 24;
+    *p++ = 0x08;
+    *p++ = 0x00;
+    
+    /* initialize all other segments */
+    *p++ = 0xb8;                /* MOV EAX,0x10 */
+    *p++ = 0x10;
+    *p++ = 0x00;
+    *p++ = 0x00;
+    *p++ = 0x00;
+    for (i = 0; i < 6; i++) {
+        if (i == 1)                /* Skip CS */
+            continue;
+
+        *p++ = 0x8e;                /* MOV <seg>,EAX */
+        *p++ = 0xc0 + (i << 3);
+    }
+
+    /* EBX contains a pointer to the bootinfo struct */
+    *p++ = 0xbb;                /* MOV EBX,imm32 */
+    *p++ = bootinfo;
+    *p++ = bootinfo >> 8;
+    *p++ = bootinfo >> 16;
+    *p++ = bootinfo >> 24;
+
+    /* EAX has to contain the following magic */
+    *p++ = 0xb8;                /* MOV EAX,0x2badb002 */
+    *p++ = 0x02;
+    *p++ = 0xb0;
+    *p++ = 0xad;
+    *p++ = 0x2b;
+
+    /* Jump off to the kernel */
+    *p++ = 0xea;                /* JMP */
+    *p++ = mh_entry_addr;        /* IP */
+    *p++ = mh_entry_addr >> 8;
+    *p++ = mh_entry_addr >> 16;
+    *p++ = mh_entry_addr >> 24;
+    *p++ = 0x08;
+    *p++ = 0x00;
+
+    { /* GDT loading */
+        uint32_t gdt_base = 0x00007c00 + (p - bootsect); // 0x00007c00 is the 
first IP;
+        uint32_t gdtr = gdt_base + 0x28;
+        uint8_t gdt[] = { // GDT base: 0x00000100
+            // 0x00
+            0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
+            // 0x08: code segment (base=0, limit=0xfffff, type=32bit code 
exec/read, DPL=0, 4k)
+            0xff, 0xff, 0x00, 0x00, 0x00, 0x9a, 0xcf, 0x00,
+            // 0x10: data segment (base=0, limit=0xfffff, type=32bit data 
read/write, DPL=0, 4k)
+            0xff, 0xff, 0x00, 0x00, 0x00, 0x92, 0xcf, 0x00,
+            // 0x18: code segment (base=0, limit=0x0ffff, type=16bit code 
exec/read/conf, DPL=0, 1b)
+            0xff, 0xff, 0x00, 0x00, 0x00, 0x9e, 0x00, 0x00,
+            // 0x20: data segment (base=0, limit=0x0ffff, type=16bit data 
read/write, DPL=0, 1b)
+            0xff, 0xff, 0x00, 0x00, 0x00, 0x92, 0x00, 0x00,
+            // 0x28: gdtdesc
+            0x27, 0x00, gdt_base, gdt_base >> 8, gdt_base >> 16, gdt_base >> 24
+        };
+
+        memcpy(p, gdt, sizeof(gdt));
+        p+=sizeof(gdt);
+        *pgdt++ = gdtr;
+        *pgdt++ = gdtr >> 8;
+        *pgdt++ = gdtr >> 16;
+        *pgdt++ = gdtr >> 24;
+    }
+    
+    fprintf(stderr, "qemu: multiboot loader code is %d bytes long.\n", 
(int)(p-bootsect));
+
+    bdrv_set_boot_sector(drives_table[hda].bdrv, bootsect, sizeof(bootsect));
+}
+
+static int load_multiboot(FILE *f,
+                          const char *kernel_filename,
+                          const char *initrd_filename,
+                          const char *kernel_cmdline,
+                          uint8_t *header)
+{
+    int i, is_multiboot = 0;
+    uint32_t flags = 0;
+    // XXX: multiboot header may be within the first 8192 bytes, but header
+    //      is only the first 1024
+        
+        
+    // Ok, let's see if it is a multiboot image
+    for(i=0; i<(256 - 12); i+=4) { // the header is 12x32bit long
+        if(ldl_p(header+i) == 0x1BADB002) {
+            uint32_t checksum = ldl_p(header+i+8);
+            flags = ldl_p(header+i+4);
+            checksum += flags;
+            checksum += (uint32_t)0x1BADB002;
+            if(!checksum) {
+                is_multiboot = 1;
+                break;
+            }
+        }
+    }
+    
+    if(!is_multiboot) return 0; // no multiboot
+    fprintf(stderr, "qemu: I believe we found a multiboot image!\n");
+
+    if(flags & 0x00000004) { // MULTIBOOT_HEADER_HAS_VBE
+        fprintf(stderr, "qemu: multiboot knows VBE. we don't.\n");
+    } 
+    if(!(flags & 0x00010000)) { // MULTIBOOT_HEADER_HAS_ADDR
+        // XXX: multiboot knows ELF. we don't.
+        fprintf(stderr, "qemu: multiboot knows ELF. we don't.\n");
+        return 0;
+    } else {
+        /* Valid if mh_flags sets MULTIBOOT_HEADER_HAS_ADDR. */
+        uint32_t mh_header_addr = ldl_p(header+i+12);
+        uint32_t mh_load_addr = ldl_p(header+i+16);
+        uint32_t mh_load_end_addr = ldl_p(header+i+20);
+        uint32_t mh_bss_end_addr = ldl_p(header+i+24);
+        uint32_t mh_entry_addr = ldl_p(header+i+28);
+        uint8_t *mb_kernel_addr = phys_ram_base + (mh_load_addr);
+        uint32_t mb_kernel_text_offset = i - (mh_header_addr - mh_load_addr);
+        uint32_t mb_kernel_size = get_file_size(f) - mb_kernel_text_offset;
+        uint32_t mb_bootinfo = 0x100000;
+        uint32_t tmp_size;
+
+        /* Valid if mh_flags sets MULTIBOOT_HEADER_HAS_VBE.
+        uint32_t mh_mode_type = ldl_p(header+i+32);
+        uint32_t mh_width = ldl_p(header+i+36);
+        uint32_t mh_height = ldl_p(header+i+40);
+        uint32_t mh_depth = ldl_p(header+i+44); */
+        
+        fprintf(stderr, "multiboot: mh_header_addr = %#x\n", mh_header_addr);
+        fprintf(stderr, "multiboot: mh_load_addr = %#x\n", mh_load_addr);
+        fprintf(stderr, "multiboot: mh_load_end_addr = %#x\n", 
mh_load_end_addr);
+        fprintf(stderr, "multiboot: mh_bss_end_addr = %#x\n", mh_bss_end_addr);
+        fprintf(stderr, "multiboot: mh_entry_addr = %#x\n", mh_entry_addr);
+        
+        fseek(f, mb_kernel_text_offset, SEEK_SET);
+
+        fprintf(stderr, "qemu: loading multiboot kernel (%#x bytes) at %#zx\n",
+                mb_kernel_size, mb_kernel_addr - phys_ram_base);
+
+        if ((tmp_size=fread(mb_kernel_addr, 1, mb_kernel_size, f)) != 
mb_kernel_size) {
+            fprintf(stderr, "qemu: read error on multiboot kernel '%s' (%#x != 
%#x)\n", kernel_filename, tmp_size, mb_kernel_size);
+            exit(1);
+        }
+        fclose(f);
+        
+        // the kernel is where we want it to be now
+        // XXX: add module support (hurd, xen)
+
+        // XXX: add support for:
+        //      - mem_lower (4), mem_upper (8)          flags[0]
+        //      - cmdline (12)                          flags[2]
+        //      - mods_count (20), mods_addr(24)        flags[3]
+        //      - syms (28 - 40)                        flags[4]
+        //      - mmap_length (44), mmmap_addr (48)     flags[6]
+        stl_p(phys_ram_base + mb_bootinfo, 2); // we only support the 
boot_device info
+        stl_p(phys_ram_base + mb_bootinfo + 12, 0x8001ffff); // XXX: use the 
-boot switch
+
+        generate_bootsect_multiboot(mh_entry_addr, mb_bootinfo);
+    }
+    
+    return 1; // yes, we are multiboot
+}
+
 static void load_linux(const char *kernel_filename,
                       const char *initrd_filename,
                       const char *kernel_cmdline)
@@ -523,10 +743,15 @@ static void load_linux(const char *kerne
 #if 0
     fprintf(stderr, "header magic: %#x\n", ldl_p(header+0x202));
 #endif
-    if (ldl_p(header+0x202) == 0x53726448)
-       protocol = lduw_p(header+0x206);
-    else
-       protocol = 0;
+    if (ldl_p(header+0x202) == 0x53726448) {
+        protocol = lduw_p(header+0x206);
+    } else {
+        // This looks like a multiboot kernel. If it is, let's stop 
+        // treating it like Linux.
+        if(load_multiboot(f,kernel_filename,initrd_filename, kernel_cmdline, 
header))
+            return;
+        protocol = 0;
+    }
 
     if (protocol < 0x200 || !(header[0x211] & 0x01)) {
        /* Low kernel */