[Qemu-devel] [PATCH v5 4/4] i386: allow to load initrd below 4G for recent linux

[Li Zhijian]

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.

CC: Paolo Bonzini <address@hidden>
CC: Richard Henderson <address@hidden>
CC: Eduardo Habkost <address@hidden>
CC: "Michael S. Tsirkin" <address@hidden>
CC: Marcel Apfelbaum <address@hidden>
Signed-off-by: Li Zhijian <address@hidden>

---
V5: udpate comments and changelog
V3: correct grammar and check XLF_CAN_BE_LOADED_ABOVE_4G first (Michael S. 
Tsirkin)

Signed-off-by: Li Zhijian <address@hidden>
---
 hw/i386/pc.c | 24 +++++++++++++++++++++++-
 1 file changed, 23 insertions(+), 1 deletion(-)

diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 89c25b2..ea7a3c7 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -904,7 +904,29 @@ 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.
+         *
+         * FIXME: it's possible that linux protocol within [0x208, 0x20c]
+         * supports up to 4G initrd as well.
+         */
+        initrd_max = UINT32_MAX;
+    } else if (protocol >= 0x203) {
         initrd_max = ldl_p(header+0x22c);
     } else {
         initrd_max = 0x37ffffff;
-- 
2.7.4