Patch "x86/kexec: Make kexec (mostly) work in 5-level paging mode" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

x86/kexec: Make kexec (mostly) work in 5-level paging mode

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-kexec-make-kexec-mostly-work-in-5-level-paging-mode.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Kirill A. Shutemov" <[email protected]>
Date: Mon, 29 Jan 2018 14:08:45 +0300
Subject: x86/kexec: Make kexec (mostly) work in 5-level paging mode

From: Kirill A. Shutemov <[email protected]>

commit 5bf30316991d5bcda046343ee77d823cf16fdd03 upstream.

Currently kexec() will crash when switching into a 5-level paging enabled kernel.

I missed that we need to change relocate_kernel() to set CR4.LA57 flag if the kernel has 5-level paging enabled.

I avoided using #ifdef CONFIG_X86_5LEVEL here and inferred if we need to enable 5-level paging from previous CR4 value. This way the code is ready for boot-time switching between paging modes.

With this patch applied, in addition to kexec 4-to-4 which always worked, we can kexec 4-to-5 and 5-to-5 - while 5-to-4 will need more work.

---
 arch/x86/kernel/relocate_kernel_64.S |    8 ++++++++
 1 file changed, 8 insertions(+)

@@ -68,6 +68,9 @@ relocate_kernel:
 	movq	%cr4, %rax
 	movq	%rax, CR4(%r11)
 
+	/* Save CR4. Required to enable the right paging mode later. */
+	movq	%rax, %r13
+
 	/* zero out flags, and disable interrupts */
 	pushq $0
 	popfq

@@ -126,8 +129,13 @@ identity_mapped:
 	/*
 	 * Set cr4 to a known state:
 	 *  - physical address extension enabled
+	 *  - 5-level paging, if it was enabled before
 	 */
 	movl	$X86_CR4_PAE, %eax
+	testq	$X86_CR4_LA57, %r13
+	jz	1f
+	orl	$X86_CR4_LA57, %eax
+1:
 	movq	%rax, %cr4
 
 	jmp 1f

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/x86-kexec-make-kexec-mostly-work-in-5-level-paging-mode.patch