[RFC PATCH 4/7] x86/kexec: Debugging support: load a GDT

[David Woodhouse]

From: David Woodhouse <dwmw at amazon.co.uk>

There are some failure modes which lead to triple-faults in the
relocate_kernel function, which is fairly much undebuggable for normal
mortals.

Adding a GDT in the relocate_kernel environment is step 1 towards being
able to catch faults and do something more useful.

Signed-off-by: David Woodhouse <dwmw at amazon.co.uk>
---
 arch/x86/kernel/relocate_kernel_64.S | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S
index 9b6ed08e00ea..2af4ce593645 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -131,6 +131,21 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
 	/* store the start address on the stack */
 	pushq   %rdx
 
+#ifdef DEBUG
+	/* Create a GDTR (16 bits limit, 64 bits addr) on stack */
+	leaq	.Lreloc_kernel_gdt(%rip), %r8
+	pushq	%r8
+	pushw	(%r8)
+
+	/* Load the GDT, put the stack back */
+	lgdt	(%rsp)
+	addq	$10, %rsp
+
+	/* Test that we can load segments */
+	movq	%ds, %rax
+	movq	%rax, %ds
+#endif /* DEBUG */
+
 	/*
 	 * Clear X86_CR4_CET (if it was set) such that we can clear CR0_WP
 	 * below.
@@ -331,5 +346,16 @@ SYM_CODE_START_LOCAL_NOALIGN(swap_pages)
 	int3
 SYM_CODE_END(swap_pages)
 
+#ifdef DEBUG
+.Lreloc_kernel_gdt:
+	.word   1f - .Lreloc_kernel_gdt - 1
+	.long   0
+	.word   0
+	.quad   0x00cf9a000000ffff      /* __KERNEL32_CS */
+	.quad   0x00af9a000000ffff      /* __KERNEL_CS */
+	.quad   0x00cf92000000ffff      /* __KERNEL_DS */
+1:
+#endif /* DEBUG */
+
 	.skip KEXEC_CONTROL_CODE_MAX_SIZE - (. - relocate_kernel), 0xcc
 SYM_CODE_END(relocate_range);
-- 
2.44.0