x86/efi: Add early thunk code to go from 64-bit to 32-bit

Implement the transition code to go from IA32e mode to protected mode in
the EFI boot stub. This is required to use 32-bit EFI services from a
64-bit kernel.

Since EFI boot stub is executed in an identity-mapped region, there's
not much we need to do before invoking the 32-bit EFI boot services.
However, we do reload the firmware's global descriptor table
(efi32_boot_gdt) in case things like timer events are still running in
the firmware.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>

1 files changed, 29 insertions, 0 deletions

diff --git a/arch/x86/boot/compressed/efi_stub_64.S b/arch/x86/boot/compressed/efi_stub_64.S
index cedc60de86eb..7ff3632806b1 100644
--- a/arch/x86/boot/compressed/efi_stub_64.S
+++ b/arch/x86/boot/compressed/efi_stub_64.S
@@ -1 +1,30 @@
+#include <asm/segment.h>
+#include <asm/msr.h>
+#include <asm/processor-flags.h>
+
 #include "../../platform/efi/efi_stub_64.S"
+
+#ifdef CONFIG_EFI_MIXED
+        .code64
+        .text
+ENTRY(efi64_thunk)
+        push    %rbp
+        push    %rbx
+
+        subq    $16, %rsp
+        leaq    efi_exit32(%rip), %rax
+        movl    %eax, 8(%rsp)
+        leaq    efi_gdt64(%rip), %rax
+        movl    %eax, 4(%rsp)
+        movl    %eax, 2(%rax)           /* Fixup the gdt base address */
+        leaq    efi32_boot_gdt(%rip), %rax
+        movl    %eax, (%rsp)
+
+        call    __efi64_thunk
+
+        addq    $16, %rsp
+        pop     %rbx
+        pop     %rbp
+        ret
+ENDPROC(efi64_thunk)
+#endif /* CONFIG_EFI_MIXED */