linuxboot: compute initrd loading address

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>

pc-bios/optionrom/linuxboot.S

@@ -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
+	mov             %esp, %ebp
-	sub		$16, %esp
+	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

pc-bios/optionrom/optionrom.h

@@ -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);		\