On Fri, Dec 21, 2018 at 11:10:30AM -0500, Michael S. Tsirkin wrote:
On Thu, Dec 06, 2018 at 10:32:13AM +0800, Li Zhijian wrote:
a new field xloadflags was added to recent x86 linux, and BIT 1:
XLF_CAN_BE_LOADED_ABOVE_4G is used to tell bootload that where initrd can be
loaded safely.
Current QEMU/BIOS always loads initrd below below_4g_mem_size which is always
less than 4G, so here limiting initrd_max to 4G - 1 simply is enough if
this bit is set.
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>
---
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 | 10 +++++++++-
1 file changed, 9 insertions(+), 1 deletion(-)
diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 3b10726..baa99c0 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -904,7 +904,15 @@ 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) {
+ /*
+ * Although kernel allows initrd loading to above 4G,
+ * it just makes it as large as possible while still staying below 4G
+ * since current BIOS always loads initrd below pcms->below_4g_mem_size
+ */
+ initrd_max = UINT32_MAX;
+ } else if (protocol >= 0x203) {
initrd_max = ldl_p(header+0x22c);
} else {
initrd_max = 0x37ffffff;
I still have trouble understanding the above.
Anyone else wants to comment / help rephrase the comment
and commit log so it's readable?
The comment seems to contradict what I see on the code:
| Although kernel allows initrd loading to above 4G,
Sounds correct.
| it just makes it as large as possible while still staying below 4G
I'm not a native English speaker, but I believe "it" here should
be interpreted as "the kernel", which would be incorrect. It's
this QEMU function that limits initrd_max to a uint32 value, not
the kernel.
| since current BIOS always loads initrd below pcms->below_4g_mem_size
I don't know why the BIOS is mentioned here. The
below_4g_mem_size limit comes from these 2 lines inside
load_linux():
if (initrd_max >= pcms->below_4g_mem_size - pcmc->acpi_data_size) {
initrd_max = pcms->below_4g_mem_size - pcmc->acpi_data_size - 1;
}
In addition to that, initrd_max is uint32_t simply because QEMU
doesn't support the 64-bit boot protocol (specifically the
ext_ramdisk_image field), so all talk about below_4g_mem_size
seems to be just a distraction.
All that said, I miss one piece of information here: is
XLF_CAN_BE_LOADED_ABOVE_4G really supposed to override
header+0x22c? linux/Documentation/x86/boot.txt isn't clear about
that. Is there any reference that can help us confirm this?