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Date: Wed, 20 Aug 2014 11:27:41 +0100
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From: Stefan Hajnoczi <stefanha@gmail.com>
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Subject: [Qemu-devel] Recent SeaBIOS too big for QEMU -initrd
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To: qemu-devel <qemu-devel@nongnu.org>
Cc: Kevin OConnor <kevin@koconnor.net>, John Snow <jsnow@redhat.com>, Gerd Hoffmann <kraxel@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>

The QEMU -initrd option loads the initrd at the top of RAM.  There is
a 64 KB safety region for ACPI tables in hw/i386/pc.c:load_linux():

  initrd_max = max_ram_size-ACPI_DATA_SIZE-1;

QEMU's bios-256k.bin SeaBIOS build reserves 128 KB at the top of
memory so the 64 KB ACPI data size has become too small.

The guest Linux kernel rejects the initrd:
BIOS-provided physical RAM map:
 BIOS-e820: 0000000000010000 - 000000000009fc00 (usable)
 BIOS-e820: 000000000009fc00 - 00000000000a0000 (reserved)
 BIOS-e820: 00000000000f0000 - 0000000000100000 (reserved)
 BIOS-e820: 0000000000100000 - 000000003ffe0000 (usable)
 BIOS-e820: 000000003ffe0000 - 0000000040000000 (reserved)
 BIOS-e820: 00000000feffc000 - 00000000ff000000 (reserved)
 BIOS-e820: 00000000fffc0000 - 0000000100000000 (reserved)
...
initrd extends beyond end of memory (0x3ffef79f > 0x3ffe0000)
disabling initrd

It is easy enough to "fix" the problem by bumping ACPI_DATA_SIZE up to
0x20000 in QEMU.  Perhaps this should only be done for bios-256k.bin
guests and not bios-128k.bin guests (QEMU 1.7 and older machine
types).

Perhaps QEMU -> SeaBIOS -> linuxboot.bin can be simplified so QEMU
doesn't have to guess what e820 region SeaBIOS will reserve.
linuxboot.bin would probably be the place to do it unless SeaBIOS has
Linux loading functionality that could be reused.

Any ideas?

Stefan