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[Qemu-devel] [PATCH 18/43] linuxboot: fix loading old kernels

From: Michael Roth
Subject: [Qemu-devel] [PATCH 18/43] linuxboot: fix loading old kernels
Date: Tue, 24 Feb 2015 15:47:53 -0600

From: Paolo Bonzini <address@hidden>

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: address@hidden
Cc: address@hidden
Cc: address@hidden
Signed-off-by: Paolo Bonzini <address@hidden>
(cherry picked from commit 269e2358492b674c50160553d037702e916b9f1b)
Signed-off-by: Michael Roth <address@hidden>
 pc-bios/linuxboot.bin         | Bin 1024 -> 1024 bytes
 pc-bios/optionrom/linuxboot.S |  37 +++++++++++++++++++++++++++----------
 2 files changed, 27 insertions(+), 10 deletions(-)

diff --git a/pc-bios/linuxboot.bin b/pc-bios/linuxboot.bin
GIT binary patch
delta 142

delta 89

diff --git a/pc-bios/optionrom/linuxboot.S b/pc-bios/optionrom/linuxboot.S
index 5bc0af0..ba821ab 100644
--- a/pc-bios/optionrom/linuxboot.S
+++ b/pc-bios/optionrom/linuxboot.S
@@ -76,7 +76,31 @@ boot_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
+       /* 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 */
        /* 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         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

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