x86/xen/efi: Initialize UEFI secure boot state during dom0 boot

Initialize UEFI secure boot state during dom0 boot. Otherwise the kernel
may not even know that it runs on secure boot enabled platform.

Note that part of drivers/firmware/efi/libstub/secureboot.c is duplicated
by this patch, only in this case, it runs in the context of the kernel
proper rather than UEFI boot context. The reason for the duplication is
that maintaining the original code to run correctly on ARM/arm64 as well
as on all the quirky x86 firmware we support is enough of a burden as it
is, and adding the x86/Xen execution context to that mix just so we can
reuse a single routine just isn't worth it.

[ardb: explain rationale for code duplication]
Signed-off-by: Daniel Kiper <daniel.kiper@oracle.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20180504060003.19618-2-ard.biesheuvel@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

drivers/firmware/efi/libstub/secureboot.c

@@ -30,6 +30,9 @@ static const efi_char16_t shim_MokSBState_name[] = L"MokSBState";
 
 /*
  * Determine whether we're in secure boot mode.
+ *
+ * Please keep the logic in sync with
+ * arch/x86/xen/efi.c:xen_efi_get_secureboot().
  */
 enum efi_secureboot_mode efi_get_secureboot(efi_system_table_t *sys_table_arg)
 {