efi/x86: Re-disable RT services for 32-bit kernels running on 64-bit EFI

Commit a8147db ("efi/x86: Rename efi_is_native() to efi_is_mixed()")
renamed and refactored efi_is_native() into efi_is_mixed(), but failed
to take into account that these are not diametrical opposites.

Mixed mode is a construct that permits 64-bit kernels to boot on 32-bit
firmware, but there is another non-native combination which is supported,
i.e., 32-bit kernels booting on 64-bit firmware, but only for boot and not
for runtime services. Also, mixed mode can be disabled in Kconfig, in
which case the 64-bit kernel can still be booted from 32-bit firmware,
but without access to runtime services.

Due to this oversight, efi_runtime_supported() now incorrectly returns
true for such configurations, resulting in crashes at boot. So fix this
by making efi_runtime_supported() aware of this.

As a side effect, some efi_thunk_xxx() stubs have become obsolete, so
remove them as well.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Arvind Sankar <nivedita@alum.mit.edu>
Cc: Matthew Garrett <mjg59@google.com>
Cc: linux-efi@vger.kernel.org
Link: https://lkml.kernel.org/r/20200103113953.9571-4-ardb@kernel.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

arch/x86/include/asm/efi.h

@@ -163,10 +163,10 @@ static inline bool efi_is_mixed(void)
 
 static inline bool efi_runtime_supported(void)
 {
-	if (!efi_is_mixed())
+	if (IS_ENABLED(CONFIG_X86_64) == efi_enabled(EFI_64BIT))
 		return true;
 
-	if (!efi_enabled(EFI_OLD_MEMMAP))
+	if (IS_ENABLED(CONFIG_EFI_MIXED) && !efi_enabled(EFI_OLD_MEMMAP))
 		return true;
 
 	return false;
@@ -176,27 +176,13 @@ extern void parse_efi_setup(u64 phys_addr, u32 data_len);
 
 extern void efifb_setup_from_dmi(struct screen_info *si, const char *opt);
 
-#ifdef CONFIG_EFI_MIXED
 extern void efi_thunk_runtime_setup(void);
 extern efi_status_t efi_thunk_set_virtual_address_map(
 	void *phys_set_virtual_address_map,
 	unsigned long memory_map_size,
 	unsigned long descriptor_size,
 	u32 descriptor_version,
 	efi_memory_desc_t *virtual_map);
-#else
-static inline void efi_thunk_runtime_setup(void) {}
-static inline efi_status_t efi_thunk_set_virtual_address_map(
-	void *phys_set_virtual_address_map,
-	unsigned long memory_map_size,
-	unsigned long descriptor_size,
-	u32 descriptor_version,
-	efi_memory_desc_t *virtual_map)
-{
-	return EFI_SUCCESS;
-}
-#endif /* CONFIG_EFI_MIXED */
-
 
 /* arch specific definitions used by the stub code */