x86/kexec: Invalidate the IDT earlier in kexec_reloc()

After switching stack, it is not safe to run any exception handlers, because
attempts to access the cpu_info block are out-of-bounds and will generate wild
accesses.

Invalidating the IDT in the common path means there's no need to do so again
in the 32bit path, so drop compat_mode_idt entirely.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>

xen/arch/x86/x86_64/kexec_reloc.S

@@ -44,6 +44,16 @@ FUNC(kexec_reloc, PAGE_SIZE)
 
         movq    %rcx, %rbp
 
+        /*
+         * Invalidate the IDT.  After switching off Xen's stacks, the
+         * exception handlers are unsafe to use, because there's no way to
+         * perform arithmetic on the stack pointer to find the cpu_info block.
+         */
+        push    $0
+        pushw   $0
+        lidt    (%rsp)
+        add     $10, %rsp
+
         /*
          * Move to the identity mapped stack.
          *
@@ -94,8 +104,6 @@ FUNC(kexec_reloc, PAGE_SIZE)
         jmp     *%rbp
 
 .L_call_32_bit:
-        /* Setup IDT. */
-        lidt    compat_mode_idt(%rip)
 
         /* Load compat GDT. */
         leaq    compat_mode_gdt(%rip), %rax
@@ -202,11 +210,6 @@ DATA_LOCAL(compat_mode_gdt, 8)
 .Lcompat_mode_gdt_end:
 END(compat_mode_gdt)
 
-DATA_LOCAL(compat_mode_idt)
-        .word 0                      /* limit */
-        .long 0                      /* base */
-END(compat_mode_idt)
-
         /*
          * 16 words of stack are more than enough.
          */