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Message-Id: <20231024192648.25527-1-bjorn@kernel.org>
Date: Tue, 24 Oct 2023 21:26:48 +0200
From: Björn Töpel <bjorn@...nel.org>
To: Albert Ou <aou@...s.berkeley.edu>,
Anup Patel <anup@...infault.org>,
Atish Patra <atishp@...shpatra.org>,
Atish Patra <atishp@...osinc.com>,
Palmer Dabbelt <palmer@...belt.com>,
Paul Walmsley <paul.walmsley@...ive.com>
Cc: Björn Töpel <bjorn@...osinc.com>,
Ard Biesheuvel <ardb@...nel.org>,
Sunil V L <sunilvl@...tanamicro.com>,
linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org
Subject: [PATCH] riscv: CONFIG_EFI should not depend on CONFIG_RISCV_ISA_C
From: Björn Töpel <bjorn@...osinc.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@...osinc.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
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