From: Joerg Roedel <jroe...@suse.de>

Handling exceptions during boot requires a working GDT. The kernel GDT
is not yet ready for use, so install a temporary boot GDT.

Signed-off-by: Joerg Roedel <jroe...@suse.de>
---
 arch/x86/kernel/head_64.S | 26 ++++++++++++++++++++++++++
 1 file changed, 26 insertions(+)

diff --git a/arch/x86/kernel/head_64.S b/arch/x86/kernel/head_64.S
index 4bbc770af632..5a3cde971cb7 100644
--- a/arch/x86/kernel/head_64.S
+++ b/arch/x86/kernel/head_64.S
@@ -72,6 +72,20 @@ SYM_CODE_START_NOALIGN(startup_64)
        /* Set up the stack for verify_cpu(), similar to initial_stack below */
        leaq    (__end_init_task - SIZEOF_PTREGS)(%rip), %rsp
 
+       /* Setup boot GDT descriptor and load boot GDT */
+       leaq    boot_gdt(%rip), %rax
+       movq    %rax, boot_gdt_base(%rip)
+       lgdt    boot_gdt_descr(%rip)
+
+       /* GDT loaded - switch to __KERNEL_CS so IRET works reliably */
+       pushq   $__KERNEL_CS
+       leaq    .Lon_kernel_cs(%rip), %rax
+       pushq   %rax
+       lretq
+
+.Lon_kernel_cs:
+       UNWIND_HINT_EMPTY
+
        /* Sanitize CPU configuration */
        call verify_cpu
 
@@ -480,6 +494,18 @@ SYM_DATA_LOCAL(early_gdt_descr_base,       .quad 
INIT_PER_CPU_VAR(gdt_page))
 SYM_DATA(phys_base, .quad 0x0)
 EXPORT_SYMBOL(phys_base)
 
+/* Boot GDT used when kernel addresses are not mapped yet */
+SYM_DATA_LOCAL(boot_gdt_descr,         .word boot_gdt_end - boot_gdt)
+SYM_DATA_LOCAL(boot_gdt_base,          .quad 0)
+SYM_DATA_START(boot_gdt)
+       .quad   0
+       .quad   0x00cf9a000000ffff      /* __KERNEL32_CS */
+       .quad   0x00af9a000000ffff      /* __KERNEL_CS */
+       .quad   0x00cf92000000ffff      /* __KERNEL_DS */
+       .quad   0x0080890000000000      /* TS descriptor */
+       .quad   0x0000000000000000      /* TS continued */
+SYM_DATA_END_LABEL(boot_gdt, SYM_L_LOCAL, boot_gdt_end)
+
 #include "../../x86/xen/xen-head.S"
 
        __PAGE_ALIGNED_BSS
-- 
2.17.1

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