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

[Dmitry Malkin]

there may be some other reasons which may cause undefined behavior 
(reboot for example):

in arch/x86/boot/compressed/pgtable_64.c in function paging_prepare():

1. structure "paging_config" allocated on stack without setting default 
value for flag "l5_required":
>>struct paging_config paging_config = {};
l5_required is set only if CONFIG_X86_5LEVEL is defined

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").

3. paging_prepare(void) returns "struct paging_config". Copy by value. 
Is it really specified by ABI or GCC itself that the second field (which 
is flag "l5_required") will go to RDX register?