From: David Woodhouse <dwmw@xxxxxxxxxxxx> 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@xxxxxxxxxxxx> --- arch/x86/kernel/relocate_kernel_64.S | 27 +++++++++++++++++++++++++++ 1 file changed, 27 insertions(+) diff --git a/arch/x86/kernel/relocate_kernel_64.S b/arch/x86/kernel/relocate_kernel_64.S index b24198eb1fe9..6c6bfdccfe6a 100644 --- a/arch/x86/kernel/relocate_kernel_64.S +++ b/arch/x86/kernel/relocate_kernel_64.S @@ -39,6 +39,18 @@ 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) +#ifdef CONFIG_KEXEC_DEBUG + .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) +#endif /* CONFIG_KEXEC_DEBUG */ + .section .text.relocate_kernel,"ax"; .code64 SYM_CODE_START_NOALIGN(relocate_kernel) @@ -112,6 +124,21 @@ SYM_CODE_START_LOCAL_NOALIGN(identity_mapped) /* store the start address on the stack */ pushq %rdx +#ifdef CONFIG_KEXEC_DEBUG + /* 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 +#endif /* CONFIG_KEXEC_DEBUG */ + /* * Clear X86_CR4_CET (if it was set) such that we can clear CR0_WP * below. -- 2.47.0
