linuxboot: fix loading old kernels

Old kernels that used high memory only allowed the initrd to be in the
first 896MB of memory.  If you load the initrd above, they complain
that "initrd extends beyond end of memory".

In order to fix this, while not breaking machines with small amounts
of memory fixed by cdebec5 (linuxboot: compute initrd loading address,
2014-10-06), we need to distinguish two cases.  If pc.c placed the
initrd at end of memory, use the new algorithm based on the e801
memory map.  If instead pc.c placed the initrd at the maximum address
specified by the bzImage, leave it there.

The only interesting part is that the low-memory info block is now
loaded very early, in real mode, and thus the 32-bit address has
to be converted into a real mode segment.  The initrd address is
also patched in the info block before entering real mode, it is
simpler that way.

This fixes booting the RHEL4.8 32-bit installation image with 1GB
of RAM.

Cc: qemu-stable@nongnu.org
Cc: mst@redhat.com
Cc: jsnow@redhat.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

pc-bios/optionrom/linuxboot.S

@@ -76,7 +76,31 @@ boot_kernel:
 
 
 copy_kernel:
-	/* Compute initrd address */
+	/* Read info block in low memory (0x10000 or 0x90000) */
+	read_fw		FW_CFG_SETUP_ADDR
+	shr		$4, %eax
+	mov		%eax, %es
+	xor		%edi, %edi
+	read_fw_blob_addr32_edi(FW_CFG_SETUP)
+
+	cmpw            $0x203, %es:0x206      // if protocol >= 0x203
+	jae             1f                     // have initrd_max
+	movl            $0x37ffffff, %es:0x22c // else assume 0x37ffffff
+1:
+
+	/* Check if using kernel-specified initrd address */
+	read_fw		FW_CFG_INITRD_ADDR
+	mov		%eax, %edi             // (load_kernel wants it in %edi)
+	read_fw		FW_CFG_INITRD_SIZE     // find end of initrd
+	add		%edi, %eax
+	xor		%es:0x22c, %eax        // if it matches es:0x22c
+	and		$-4096, %eax           // (apart from padding for page)
+	jz		load_kernel            // then initrd is not at top
+					       // of memory
+
+	/* pc.c placed the initrd at end of memory.  Compute a better
+	 * initrd address based on e801 data.
+	 */
 	mov		$0xe801, %ax
 	xor		%cx, %cx
 	xor		%dx, %dx
@@ -107,7 +131,9 @@ copy_kernel:
 	read_fw         FW_CFG_INITRD_SIZE
 	subl            %eax, %edi
 	andl            $-4096, %edi          /* EDI = start of initrd */
+	movl		%edi, %es:0x218       /* put it in the header */
 
+load_kernel:
 	/* 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. */
@@ -139,19 +165,10 @@ 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_CMDLINE)
 
-	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
 	mov		%eax, %cr0