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

["Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>]

On Wed, Jul 25, 2018 at 05:26:02PM +0000, Dmitry Malkin wrote:
> 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

Hm? C99 initializer zeros the structure.

> 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").

Could you show dmesg from such system?

> 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?

https://github.com/hjl-tools/x86-psABI/wiki/x86-64-psABI-1.0.pdf

3.2.3 Parameter Passing

...

Returning of Values
The returning of values is done according to the following algorithm:

...

3.  If the class is INTEGER, the next available register of the sequence
%rax, %rdx is used.

-- 
 Kirill A. Shutemov