efi/x86: Move x86 back to libstub

This reverts commit 84be880560, which itself reverted my original
attempt to move x86 from #include'ing .c files from across the tree
to using the EFI stub built as a static library.

The issue that affected the original approach was that splitting
the implementation into several .o files resulted in the variable
'efi_early' becoming a global with external linkage, which under
-fPIC implies that references to it must go through the GOT. However,
dealing with this additional GOT entry turned out to be troublesome
on some EFI implementations. (GCC's visibility=hidden attribute is
supposed to lift this requirement, but it turned out not to work on
the 32-bit build.)

Instead, use a pure getter function to get a reference to efi_early.
This approach results in no additional GOT entries being generated,
so there is no need for any changes in the early GOT handling.

Tested-by: Maarten Lankhorst <maarten.lankhorst@canonical.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>

arch/x86/boot/compressed/Makefile

@@ -35,7 +35,8 @@ vmlinux-objs-$(CONFIG_RANDOMIZE_BASE) += $(obj)/aslr.o
 
 $(obj)/eboot.o: KBUILD_CFLAGS += -fshort-wchar -mno-red-zone
 
-vmlinux-objs-$(CONFIG_EFI_STUB) += $(obj)/eboot.o $(obj)/efi_stub_$(BITS).o
+vmlinux-objs-$(CONFIG_EFI_STUB) += $(obj)/eboot.o $(obj)/efi_stub_$(BITS).o \
+	$(objtree)/drivers/firmware/efi/libstub/lib.a
 
 $(obj)/vmlinux: $(vmlinux-objs-y) FORCE
 	$(call if_changed,ld)

arch/x86/boot/compressed/eboot.c

@@ -21,8 +21,10 @@ static efi_system_table_t *sys_table;
 
 static struct efi_config *efi_early;
 
-#define efi_call_early(f, ...)						\
-	efi_early->call(efi_early->f, __VA_ARGS__);
+__pure const struct efi_config *__efi_early(void)
+{
+	return efi_early;
+}
 
 #define BOOT_SERVICES(bits)						\
 static void setup_boot_services##bits(struct efi_config *c)		\
@@ -285,8 +287,6 @@ void efi_char16_printk(efi_system_table_t *table, efi_char16_t *str)
 	}
 }
 
-#include "../../../../drivers/firmware/efi/libstub/efi-stub-helper.c"
-
 static void find_bits(unsigned long mask, u8 *pos, u8 *size)
 {
 	u8 first, len;

arch/x86/boot/compressed/eboot.h

@@ -103,20 +103,4 @@ struct efi_uga_draw_protocol {
 	void *blt;
 };
 
-struct efi_config {
-	u64 image_handle;
-	u64 table;
-	u64 allocate_pool;
-	u64 allocate_pages;
-	u64 get_memory_map;
-	u64 free_pool;
-	u64 free_pages;
-	u64 locate_handle;
-	u64 handle_protocol;
-	u64 exit_boot_services;
-	u64 text_output;
-	efi_status_t (*call)(unsigned long, ...);
-	bool is64;
-} __packed;
-
 #endif /* BOOT_COMPRESSED_EBOOT_H */