Commit aab50e53 authored Jan 17, 2019 by Li Zhijian Committed by Paolo Bonzini Feb 05, 2019

i386: allow to load initrd below 4 GB for recent linux



Since linux commit: cf8fa920cb42 ("i386: handle an initrd in highmem (version 2)")
linux has supported initrd up to 4 GB, but the header field
ramdisk_max is still set to 2 GB to avoid "possible bootloader bugs".

When use '-kernel vmlinux -initrd initrd.cgz' to launch a VM,
the firmware(it could be linuxboot_dma.bin) helps to read initrd
contents into guest memory(below ramdisk_max) and jump to kernel.
that's similar with what bootloader does, like grub.

In addition, initrd_max is uint32_t simply because QEMU doesn't support
the 64-bit boot protocol (specifically the ext_ramdisk_image field).

Therefore here just limit initrd_max to UINT32_MAX simply as well to
allow initrd to be loaded below 4 GB.

NOTE: it's possible that linux protocol within [0x208, 0x20c]
supports up to 4 GB initrd as well.

CC: Paolo Bonzini <pbonzini@redhat.com>
CC: Richard Henderson <rth@twiddle.net>
CC: Eduardo Habkost <ehabkost@redhat.com>
CC: "Michael S. Tsirkin" <mst@redhat.com>
CC: Marcel Apfelbaum <marcel.apfelbaum@gmail.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>

Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

parent 06e0259a

hw/i386/pc.c

+20 −1

Original line number	Diff line number	Diff line
		@@ -1299,7 +1299,26 @@ static void load_linux(PCMachineState *pcms,
		#endif

		/* highest address for loading the initrd */
		if (protocol >= 0x203) {
		if (protocol >= 0x20c &&
		lduw_p(header+0x236) & XLF_CAN_BE_LOADED_ABOVE_4G) {
		/*
		* Linux has supported initrd up to 4 GB for a very long time (2007,
		* long before XLF_CAN_BE_LOADED_ABOVE_4G which was added in 2013),
		* though it only sets initrd_max to 2 GB to "work around bootloader
		* bugs". Luckily, QEMU firmware(which does something like bootloader)
		* has supported this.
		*
		* It's believed that if XLF_CAN_BE_LOADED_ABOVE_4G is set, initrd can
		* be loaded into any address.
		*
		* In addition, initrd_max is uint32_t simply because QEMU doesn't
		* support the 64-bit boot protocol (specifically the ext_ramdisk_image
		* field).
		*
		* Therefore here just limit initrd_max to UINT32_MAX simply as well.
		*/
		initrd_max = UINT32_MAX;
		} else if (protocol >= 0x203) {
		initrd_max = ldl_p(header+0x22c);
		} else {
		initrd_max = 0x37ffffff;