Re: 4.17.x won't boot due to "x86/boot/compressed/64: Handle 5-level paging boot if kernel is above 4G"

From: Dmitry Malkin
Date: Thu Jul 26 2018 - 12:21:49 EST

On 07/26/2018 04:50 PM, Kirill A. Shutemov wrote:
2. reading from memory which may be reserved in case of EFI systems:
ÂÂ ebda_start = *(unsigned short *)0x40e << 4;
ÂÂ bios_start = *(unsigned short *)0x413 << 10;
Also, on EFI system without CSM it will results in all zeros. Which will
place trampoline_start to 0x9d000. And it also may be reserved memory. In
fact I have such system and it is causes instant reboot (when code starts
copying to "trampoline_start").
Could you show dmesg from such system?
Sure, here it is (please note than not both pages are reserved but only
second one: 0x9e000-0x9ffff):
Well. That's bad.

I don't see much options but parse e820 in decompression code. I hoped to
avoid this.

Let me see what I can do there.
Just in case of UEFI (I don't know much about BIOS and kexec):
register RSI (right before call paging_prepare) will contains pointer to "struct boot_params" (returned by function efi_main() in eboot.c).
There are fields e820_table and e820_entries.