5 files changed, 37 insertions, 7 deletions

diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
index 0af5250d914f..8453fe1342ea 100644
--- a/arch/x86/Kconfig
+++ b/arch/x86/Kconfig
@@ -1585,6 +1585,20 @@ config EFI_STUB
 
           See Documentation/efi-stub.txt for more information.
 
+config EFI_MIXED
+        bool "EFI mixed-mode support"
+        depends on EFI_STUB && X86_64
+        ---help---
+           Enabling this feature allows a 64-bit kernel to be booted
+           on a 32-bit firmware, provided that your CPU supports 64-bit
+           mode.
+
+           Note that it is not possible to boot a mixed-mode enabled
+           kernel via the EFI boot stub - a bootloader that supports
+           the EFI handover protocol must be used.
+
+           If unsure, say N.
+
 config SECCOMP
         def_bool y
         prompt "Enable seccomp to safely compute untrusted bytecode"
diff --git a/arch/x86/boot/header.S b/arch/x86/boot/header.S
index d69d96653325..256388260c88 100644
--- a/arch/x86/boot/header.S
+++ b/arch/x86/boot/header.S
@@ -283,7 +283,7 @@ _start:
         # Part 2 of the header, from the old setup.S
 
                 .ascii  "HdrS"          # header signature
-                .word   0x020c          # header version number (>= 0x0105)
+                .word   0x020d          # header version number (>= 0x0105)
                                         # or else old loadlin-1.5 will fail)
                 .globl realmode_swtch
 realmode_swtch: .word   0, 0            # default_switch, SETUPSEG
@@ -375,7 +375,8 @@ xloadflags:
 # define XLF0 0
 #endif
 
-#if defined(CONFIG_RELOCATABLE) && defined(CONFIG_X86_64)
+#if defined(CONFIG_RELOCATABLE) && defined(CONFIG_X86_64) && \
+        !defined(CONFIG_EFI_MIXED)
    /* kernel/boot_param/ramdisk could be loaded above 4g */
 # define XLF1 XLF_CAN_BE_LOADED_ABOVE_4G
 #else
@@ -383,10 +384,14 @@ xloadflags:
 #endif
 
 #ifdef CONFIG_EFI_STUB
-# ifdef CONFIG_X86_64
-#  define XLF23 XLF_EFI_HANDOVER_64             /* 64-bit EFI handover ok */
+# ifdef CONFIG_EFI_MIXED
+#  define XLF23 (XLF_EFI_HANDOVER_32|XLF_EFI_HANDOVER_64)
 # else
-#  define XLF23 XLF_EFI_HANDOVER_32             /* 32-bit EFI handover ok */
+#  ifdef CONFIG_X86_64
+#   define XLF23 XLF_EFI_HANDOVER_64            /* 64-bit EFI handover ok */
+#  else
+#   define XLF23 XLF_EFI_HANDOVER_32            /* 32-bit EFI handover ok */
+#  endif
 # endif
 #else
 # define XLF23 0
diff --git a/arch/x86/include/asm/efi.h b/arch/x86/include/asm/efi.h
index 85935e6f69f3..a3837254e320 100644
--- a/arch/x86/include/asm/efi.h
+++ b/arch/x86/include/asm/efi.h
@@ -152,6 +152,17 @@ static inline bool efi_is_native(void)
         return IS_ENABLED(CONFIG_X86_64) == efi_enabled(EFI_64BIT);
 }
 
+static inline bool efi_runtime_supported(void)
+{
+        if (efi_is_native())
+                return true;
+
+        if (IS_ENABLED(CONFIG_EFI_MIXED) && !efi_enabled(EFI_OLD_MEMMAP))
+                return true;
+
+        return false;
+}
+
 extern struct console early_efi_console;
 extern void parse_efi_setup(u64 phys_addr, u32 data_len);
 
diff --git a/arch/x86/kernel/setup.c b/arch/x86/kernel/setup.c
index 06853e670354..ff9b3a62a05c 100644
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -1243,7 +1243,7 @@ void __init setup_arch(char **cmdline_p)
          * mismatched firmware/kernel archtectures since there is no
          * support for runtime services.
          */
-        if (efi_enabled(EFI_BOOT) && !efi_is_native()) {
+        if (efi_enabled(EFI_BOOT) && !efi_runtime_supported()) {
                 pr_info("efi: Setup done, disabling due to 32/64-bit mismatch\n");
                 efi_unmap_memmap();
         }
diff --git a/arch/x86/platform/efi/efi.c b/arch/x86/platform/efi/efi.c
index 39f5b7bba695..395decedfa2b 100644
--- a/arch/x86/platform/efi/efi.c
+++ b/arch/x86/platform/efi/efi.c
@@ -802,7 +802,7 @@ void __init efi_init(void)
          * that doesn't match the kernel 32/64-bit mode.
          */
 
-        if (!efi_is_native())
+        if (!efi_runtime_supported())
                 pr_info("No EFI runtime due to 32/64-bit mismatch with kernel\n");
         else {
                 if (disable_runtime || efi_runtime_init())