[PATCH] riscv: CONFIG_EFI should not depend on CONFIG_RISCV_ISA_C

[Björn Töpel]

From: Björn Töpel <bjorn at rivosinc.com>

UEFI/PE mandates that the kernel Image starts with "MZ" ASCII
(0x5A4D). Convenient enough, "MZ" is a valid compressed RISC-V
instruction. This means that a non-UEFI loader can simply jump to
"code0" in the Image header [1] and start executing.

The Image specification [1] says the following about "code0":
  |   This header is also reused to support EFI stub for RISC-V. EFI
  |   specification needs PE/COFF image header in the beginning of the
  |   kernel image in order to load it as an EFI application. In order
  |   to support EFI stub, code0 is replaced with "MZ" magic string
  |   and res3(at offset 0x3c) points to the rest of the PE/COFF
  |   header.

"MZ" is not a valid instruction for implementations without the C
extension.

A non-UEFI loader, loading a non-C UEFI image have the following
options:
  1. Trap and emulate "code0"
  2. Avoid "code0" if it is "MZ", and have the kernel entry at
     "code1".

Replace the compressed instruction with a hex code variant, that works
for CONFIG_RISCV_ISA_C=n builds. Further, this change also make sure
that the "code0" instruction is 32b aligned.

[1] Documentation/riscv/boot-image-header.rst

Signed-off-by: Björn Töpel <bjorn at rivosinc.com>
---
 arch/riscv/Kconfig       | 1 -
 arch/riscv/kernel/head.S | 8 ++++++--
 2 files changed, 6 insertions(+), 3 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index d607ab0f7c6d..9c5bbbc93951 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -799,7 +799,6 @@ config EFI
 	select EFI_RUNTIME_WRAPPERS
 	select EFI_STUB
 	select LIBFDT
-	select RISCV_ISA_C
 	select UCS2_STRING
 	help
 	  This option provides support for runtime services provided
diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
index 3710ea5d160f..33d69b569843 100644
--- a/arch/riscv/kernel/head.S
+++ b/arch/riscv/kernel/head.S
@@ -27,9 +27,13 @@ ENTRY(_start)
 	 */
 #ifdef CONFIG_EFI
 	/*
-	 * This instruction decodes to "MZ" ASCII required by UEFI.
+	 * The compressed (C extension) "c.li s4,-13" instruction
+	 * decodes to 0x5a4d/"MZ" (ASCII), which is required by UEFI.
+	 *
+	 * In order to support non-compressed EFI kernels, the
+	 * instruction is written in hex.
 	 */
-	c.li s4,-13
+	.word 0x5a4d5a4d
 	j _start_kernel
 #else
 	/* jump to start kernel */

base-commit: d88520ad73b79e71e3ddf08de335b8520ae41c5c
-- 
2.40.1