From: David Woodhouse <d...@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 <d...@amazon.co.uk>
---
arch/x86/kernel/relocate_kernel_64.S | 23 +++++++++++++++++++++++
1 file changed, 23 insertions(+)
diff --git a/arch/x86/kernel/relocate_kernel_64.S
b/arch/x86/kernel/relocate_kernel_64.S
index b44d8863e57f..ac058971a382 100644
--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -40,6 +40,16 @@ SYM_DATA(kexec_pa_table_page, .quad 0)
SYM_DATA(kexec_pa_swap_page, .quad 0)
SYM_DATA_LOCAL(pa_backup_pages_map, .quad 0)
+ .balign 16
+SYM_DATA_START_LOCAL(kexec_debug_gdt)
+ .word kexec_debug_gdt_end - kexec_debug_gdt - 1
+ .long 0
+ .word 0
+ .quad 0x00cf9a000000ffff /* __KERNEL32_CS */
+ .quad 0x00af9a000000ffff /* __KERNEL_CS */
+ .quad 0x00cf92000000ffff /* __KERNEL_DS */
+SYM_DATA_END_LABEL(kexec_debug_gdt, SYM_L_LOCAL, kexec_debug_gdt_end)
+
.section .text..relocate_kernel,"ax";
.code64
SYM_CODE_START_NOALIGN(relocate_kernel)
@@ -116,6 +126,19 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped)
/* store the start address on the stack */
pushq %rdx
+ /* Create a GDTR (16 bits limit, 64 bits addr) on stack */
+ leaq kexec_debug_gdt(%rip), %rax
+ pushq %rax
+ pushw (%rax)
+
+ /* Load the GDT, put the stack back */
+ lgdt (%rsp)
+ addq $10, %rsp
+
+ /* Test that we can load segments */
+ movq %ds, %rax
+ movq %rax, %ds
+
/*
* Clear X86_CR4_CET (if it was set) such that we can clear CR0_WP
* below.
--
2.48.1
