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

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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:
>>    ebda_start = *(unsigned short *)0x40e << 4;
>>    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?

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