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linuxboot: compute initrd loading address

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Even though hw/i386/pc.c tries to compute a valid loading address for the
initrd, close to the top of RAM, this does not take into account other
data that is malloced into that memory by SeaBIOS.

Luckily we can easily look at the memory map to find out how much memory is
used up there.  This patch places the initrd in the first four gigabytes,
below the first hole (as returned by INT 15h, AX=e801h).

Without this patch:
[    0.000000] init_memory_mapping: [mem 0x07000000-0x07fdffff]
[    0.000000] RAMDISK: [mem 0x0710a000-0x07fd7fff]

With this patch:
[    0.000000] init_memory_mapping: [mem 0x07000000-0x07fdffff]
[    0.000000] RAMDISK: [mem 0x07112000-0x07fdffff]

So linuxboot is able to use the 64k that were added as padding for
QEMU <= 2.1.

Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
This commit is contained in:
Paolo Bonzini 2014-10-06 16:49:57 +02:00
parent fc02086b5a
commit cdebec5e40
3 changed files with 61 additions and 7 deletions
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BIN
pc-bios/linuxboot.bin
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Binary file not shown.

47
pc-bios/optionrom/linuxboot.S
View file

@ -76,14 +76,45 @@ boot_kernel:
copy_kernel:
/* Compute initrd address */
mov $0xe801, %ax
xor %cx, %cx
xor %dx, %dx
int $0x15
/* Output could be in AX/BX or CX/DX */
or %cx, %cx
jnz 1f
or %dx, %dx
jnz 1f
mov %ax, %cx
mov %bx, %dx
1:
or %dx, %dx
jnz 2f
addw $1024, %cx /* add 1 MB */
movzwl %cx, %edi
shll $10, %edi /* convert to bytes */
jmp 3f
2:
addw $16777216 >> 16, %dx /* add 16 MB */
movzwl %dx, %edi
shll $16, %edi /* convert to bytes */
3:
read_fw FW_CFG_INITRD_SIZE
subl %eax, %edi
andl $-4096, %edi /* EDI = start of initrd */
/* We need to load the kernel into memory we can't access in 16 bit
mode, so let's get into 32 bit mode, write the kernel and jump
back again. */
/* Reserve space on the stack for our GDT descriptor. */
mov %esp, %ebp
sub $16, %esp
mov %esp, %ebp
sub $16, %esp
/* Now create the GDT descriptor */
movw $((3 * 8) - 1), -16(%bp)
@ -108,10 +139,18 @@ copy_kernel:
/* We're now running in 16-bit CS, but 32-bit ES! */
/* Load kernel and initrd */
pushl %edi
read_fw_blob_addr32_edi(FW_CFG_INITRD)
read_fw_blob_addr32(FW_CFG_KERNEL)
read_fw_blob_addr32(FW_CFG_INITRD)
read_fw_blob_addr32(FW_CFG_CMDLINE)
read_fw_blob_addr32(FW_CFG_SETUP)
read_fw FW_CFG_SETUP_ADDR
mov %eax, %edi
mov %eax, %ebx
read_fw_blob_addr32_edi(FW_CFG_SETUP)
/* Update the header with the initrd address we chose above */
popl %es:0x218(%ebx)
/* And now jump into Linux! */
mov $0, %eax

21
pc-bios/optionrom/optionrom.h
View file

@ -51,8 +51,6 @@
.endm
#define read_fw_blob_pre(var) \
read_fw var ## _ADDR; \
mov %eax, %edi; \
read_fw var ## _SIZE; \
mov %eax, %ecx; \
mov $var ## _DATA, %ax; \
@ -68,6 +66,8 @@
* Clobbers: %eax, %edx, %es, %ecx, %edi
*/
#define read_fw_blob(var) \
read_fw var ## _ADDR; \
mov %eax, %edi; \
read_fw_blob_pre(var); \
/* old as(1) doesn't like this insn so emit the bytes instead: \
rep insb (%dx), %es:(%edi); \
@ -80,7 +80,22 @@
*
* Clobbers: %eax, %edx, %es, %ecx, %edi
*/
#define read_fw_blob_addr32(var) \
#define read_fw_blob_addr32(var) \
read_fw var ## _ADDR; \
mov %eax, %edi; \
read_fw_blob_pre(var); \
/* old as(1) doesn't like this insn so emit the bytes instead: \
addr32 rep insb (%dx), %es:(%edi); \
*/ \
.dc.b 0x67,0xf3,0x6c
/*
* Read a blob from the fw_cfg device in forced addr32 mode, address is in %edi.
* Requires _SIZE and _DATA values for the parameter.
*
* Clobbers: %eax, %edx, %edi, %es, %ecx
*/
#define read_fw_blob_addr32_edi(var) \
read_fw_blob_pre(var); \
/* old as(1) doesn't like this insn so emit the bytes instead: \
addr32 rep insb (%dx), %es:(%edi); \