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Subjectinstant reboot caused by 194a9749c73d650c0
Hi Kirill

For some reason, my hosts instantly crash at boot time, with absolutely no log on console.

Bisection pointed to :

$ git bisect bad
194a9749c73d650c0b1dfdee04fb0bdf0a888ba8 is the first bad commit
commit 194a9749c73d650c0b1dfdee04fb0bdf0a888ba8
Author: Kirill A. Shutemov <>
Date: Mon Mar 12 13:02:46 2018 +0300

x86/boot/compressed/64: Handle 5-level paging boot if kernel is above 4G

This patch addresses a shortcoming in current boot process on machines
that supports 5-level paging.

If a bootloader enables 64-bit mode with 4-level paging, we might need to
switch over to 5-level paging. The switching requires the disabling
paging. It works fine if kernel itself is loaded below 4G.

But if the bootloader put the kernel above 4G (not sure if anybody does
this), we would lose control as soon as paging is disabled, because the
code becomes unreachable to the CPU.

This patch implements a trampoline in lower memory to handle this

We only need the memory for a very short time, until the main kernel
image sets up own page tables.

We go through the trampoline even if we don't have to: if we're already
in 5-level paging mode or if we don't need to switch to it. This way the
trampoline gets tested on every boot.

Signed-off-by: Kirill A. Shutemov <>
Cc: Andy Lutomirski <>
Cc: Andy Lutomirski <>
Cc: Borislav Petkov <>
Cc: Borislav Petkov <>
Cc: Brian Gerst <>
Cc: Cyrill Gorcunov <>
Cc: Denys Vlasenko <>
Cc: H. Peter Anvin <>
Cc: Josh Poimboeuf <>
Cc: Linus Torvalds <>
Cc: Matthew Wilcox <>
Cc: Peter Zijlstra <>
Cc: Thomas Gleixner <>
Signed-off-by: Ingo Molnar <>

Reverting this patch solves the problem for me.


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