[PATCHv7 5/4] x86/boot/compressed/64: Support switching from 5- to 4-level paging

[Kirill A. Shutemov]

If a bootloader enabled 5-level paging before handing off control to
kernel, we may want to switch it to 4-level paging when kernel is
compiled with CONFIG_X86_5LEVEL=n.

Let's modify decompression code to handle the situation.

This will fail if the kernel image is loaded above 64TiB since 4-level
paging would not be able to access the image.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 arch/x86/boot/compressed/head_64.S | 17 +++++++++++++++--
 arch/x86/boot/compressed/pgtable.h |  2 +-
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
index f5ac9a6515ef..5942b7d9d6a2 100644
--- a/arch/x86/boot/compressed/head_64.S
+++ b/arch/x86/boot/compressed/head_64.S
@@ -520,19 +520,32 @@ ENTRY(trampoline_32bit_src)
 	btrl	$X86_CR0_PG_BIT, %eax
 	movl	%eax, %cr0
 
-	/* For 5-level paging, point CR3 to the trampoline's new top level page table */
+	/* Check what paging mode we want to be in after the trampoline */
 	cmpl	$0, %edx
 	jz	1f
 
 	/* Don't touch CR3 if it already points to 5-level page tables */
 	movl	%cr4, %eax
 	testl	$X86_CR4_LA57, %eax
-	jnz	1f
+	jnz	2f
 
+	/* For 5-level paging, point CR3 to the trampoline's new top level page table */
 	leal	TRAMPOLINE_32BIT_PGTABLE_OFFSET(%ecx), %eax
 	movl	%eax, %cr3
 1:
+	/* Don't touch CR3 if it already points to 4-level page tables */
+	movl	%cr4, %eax
+	testl	$X86_CR4_LA57, %eax
+	jz	2f
 
+	/*
+	 * We are in 5-level paging mode, but we want to switch to 4-level.
+	 * Let's take the first entry in the top-level page table as our new CR3.
+	 */
+	movl	%cr3, %eax
+	movl	(%eax), %eax
+	movl	%eax, %cr3
+2:
 	/* Enable PAE and LA57 (if required) paging modes */
 	movl	%cr4, %eax
 	orl	$X86_CR4_PAE, %eax
diff --git a/arch/x86/boot/compressed/pgtable.h b/arch/x86/boot/compressed/pgtable.h
index 6e0db2260147..cd62c546afd5 100644
--- a/arch/x86/boot/compressed/pgtable.h
+++ b/arch/x86/boot/compressed/pgtable.h
@@ -6,7 +6,7 @@
 #define TRAMPOLINE_32BIT_PGTABLE_OFFSET	0
 
 #define TRAMPOLINE_32BIT_CODE_OFFSET	PAGE_SIZE
-#define TRAMPOLINE_32BIT_CODE_SIZE	0x60
+#define TRAMPOLINE_32BIT_CODE_SIZE	0x70
 
 #define TRAMPOLINE_32BIT_STACK_END	TRAMPOLINE_32BIT_SIZE
 
-- 
2.15.1