efi/libstub/x86: Use Exit() boot service to exit the stub on errors

Currently, we either return with an error [from efi_pe_entry()] or
enter a deadloop [in efi_main()] if any fatal errors occur during
execution of the EFI stub. Let's switch to calling the Exit() EFI boot
service instead in both cases, so that we
a) can get rid of the deadloop, and simply return to the boot manager
   if any errors occur during execution of the stub, including during
   the call to ExitBootServices(),
b) can also return cleanly from efi_pe_entry() or efi_main() in mixed
   mode, once we introduce support for LoadImage/StartImage based mixed
   mode in the next patch.

Note that on systems running downstream GRUBs [which do not use LoadImage
or StartImage to boot the kernel, and instead, pass their own image
handle as the loaded image handle], calling Exit() will exit from GRUB
rather than from the kernel, but this is a tolerable side effect.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

arch/x86/include/asm/efi.h

@@ -270,6 +270,11 @@ static inline void *efi64_zero_upper(void *p)
 	return p;
 }
 
+static inline u32 efi64_convert_status(efi_status_t status)
+{
+	return (u32)(status | (u64)status >> 32);
+}
+
 #define __efi64_argmap_free_pages(addr, size)				\
 	((addr), 0, (size))
 
@@ -288,6 +293,9 @@ static inline void *efi64_zero_upper(void *p)
 #define __efi64_argmap_locate_device_path(protocol, path, handle)	\
 	((protocol), (path), efi64_zero_upper(handle))
 
+#define __efi64_argmap_exit(handle, status, size, data)			\
+	((handle), efi64_convert_status(status), (size), (data))
+
 /* PCI I/O */
 #define __efi64_argmap_get_location(protocol, seg, bus, dev, func)	\
 	((protocol), efi64_zero_upper(seg), efi64_zero_upper(bus),	\