[PATCH v2 1/8] riscv: Add PLATFORM_MAY_SUPPORT_RISCV_ISA_C Kconfig option

[Charlie Jenkins <charlie@rivosinc.com>]

Introduce a "Kernel ISA" menu and migrate the compressed instruction
support options into a new file Kconfig.isa. Add a new option
"PLATFORM_MAY_SUPPORT_RISCV_ISA_C" that can be used to conditionally
emit C extensions if the hardware supports it.

The existing "RISCV_ISA_C" option is repurposed to be used to by kernel
code to determine if either PLATFORM_MAY_SUPPORT_RISCV_ISA_C or
PLATFORM_SUPPORTS_RISCV_ISA_C has been set.

Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
---
 arch/riscv/Kconfig                 | 19 +++++++-----------
 arch/riscv/Kconfig.isa             | 41 ++++++++++++++++++++++++++++++++++++++
 arch/riscv/Makefile                |  4 ++--
 arch/riscv/kernel/probes/uprobes.c |  2 +-
 arch/riscv/net/bpf_jit.h           |  4 +++-
 5 files changed, 54 insertions(+), 16 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index c8bdfd33abf4..c2a4f5364707 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -369,6 +369,12 @@ config ARCH_RV64I
 
 endchoice
 
+menu "Kernel ISA"
+
+source "arch/riscv/Kconfig.isa"
+
+endmenu
+
 # We must be able to map all physical memory into the kernel, but the compiler
 # is still a bit more efficient when generating code if it's setup in a manner
 # such that it can only map 2GiB of memory.
@@ -478,17 +484,6 @@ config RISCV_ALTERNATIVE_EARLY
 	help
 	  Allows early patching of the kernel for special errata
 
-config RISCV_ISA_C
-	bool "Emit compressed instructions when building Linux"
-	default y
-	help
-	  Adds "C" to the ISA subsets that the toolchain is allowed to emit
-	  when building Linux, which results in compressed instructions in the
-	  Linux binary. This option produces a kernel that will not run on
-	  systems that do not support compressed instructions.
-
-	  If you don't know what to do here, say Y.
-
 config RISCV_ISA_SVNAPOT
 	bool "Svnapot extension support for supervisor mode NAPOT pages"
 	depends on 64BIT && MMU
@@ -937,6 +932,7 @@ config EFI
 	bool "UEFI runtime support"
 	depends on OF && !XIP_KERNEL
 	depends on MMU
+	depends on PLATFORM_SUPPORTS_RISCV_ISA_C
 	default y
 	select ARCH_SUPPORTS_ACPI if 64BIT
 	select EFI_GENERIC_STUB
@@ -944,7 +940,6 @@ 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/Kconfig.isa b/arch/riscv/Kconfig.isa
new file mode 100644
index 000000000000..08b7af5aabb0
--- /dev/null
+++ b/arch/riscv/Kconfig.isa
@@ -0,0 +1,41 @@
+config RISCV_ISA_C
+	bool
+
+choice
+	prompt "Compressed instruction support"
+	default PLATFORM_SUPPORTS_RISCV_ISA_C
+	help
+	  This selects the level of support for compressed instructions to be
+	  built into the Linux Kernel. This does not impact whether compressed
+	  instructions are allowed to be emitted by user-space code.
+
+config PROHIBIT_RISCV_ISA_C
+	bool "Prohibit compressed instructions"
+	depends on NONPORTABLE
+	help
+	  Regardless of if the platform supports compressed instructions,
+	  prohibit the kernel from emitting compressed instructions.
+
+config PLATFORM_MAY_SUPPORT_RISCV_ISA_C
+	bool "Allow compressed instructions sequences if supported"
+	depends on FPU
+	depends on NONPORTABLE
+	select RISCV_ISA_C
+	help
+          Only allow compressed instructions to be emitted if "C" is present in
+          the device tree or ACPI table. No compressed instructions will be
+          emitted if the platform does not support them.
+
+config PLATFORM_SUPPORTS_RISCV_ISA_C
+	bool "Emit compressed instructions when building Linux"
+	depends on FPU
+	select RISCV_ISA_C
+	help
+	  Adds "C" to the ISA subsets that the toolchain is allowed to emit
+	  when building Linux, which results in compressed instructions in the
+	  Linux binary. This option produces a kernel that will not run on
+	  systems that do not support compressed instructions.
+
+	  If you don't know what to do here, say Y.
+
+endchoice
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 5b3115a19852..e1be36004097 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -14,7 +14,7 @@ endif
 ifeq ($(CONFIG_DYNAMIC_FTRACE),y)
 	LDFLAGS_vmlinux += --no-relax
 	KBUILD_CPPFLAGS += -DCC_USING_PATCHABLE_FUNCTION_ENTRY
-ifeq ($(CONFIG_RISCV_ISA_C),y)
+ifeq ($(CONFIG_PLATFORM_SUPPORTS_RISCV_ISA_C),y)
 	CC_FLAGS_FTRACE := -fpatchable-function-entry=4
 else
 	CC_FLAGS_FTRACE := -fpatchable-function-entry=2
@@ -65,7 +65,7 @@ endif
 riscv-march-$(CONFIG_ARCH_RV32I)	:= rv32ima
 riscv-march-$(CONFIG_ARCH_RV64I)	:= rv64ima
 riscv-march-$(CONFIG_FPU)		:= $(riscv-march-y)fd
-riscv-march-$(CONFIG_RISCV_ISA_C)	:= $(riscv-march-y)c
+riscv-march-$(CONFIG_PLATFORM_SUPPORTS_RISCV_ISA_C)	:= $(riscv-march-y)c
 riscv-march-$(CONFIG_RISCV_ISA_V)	:= $(riscv-march-y)v
 
 ifdef CONFIG_TOOLCHAIN_NEEDS_OLD_ISA_SPEC
diff --git a/arch/riscv/kernel/probes/uprobes.c b/arch/riscv/kernel/probes/uprobes.c
index 4b3dc8beaf77..a468689a6f6d 100644
--- a/arch/riscv/kernel/probes/uprobes.c
+++ b/arch/riscv/kernel/probes/uprobes.c
@@ -11,7 +11,7 @@
 
 bool is_swbp_insn(uprobe_opcode_t *insn)
 {
-#ifdef CONFIG_RISCV_ISA_C
+#ifdef CONFIG_PLATFORM_SUPPORTS_RISCV_ISA_C
 	return (*insn & 0xffff) == UPROBE_SWBP_INSN;
 #else
 	return *insn == UPROBE_SWBP_INSN;
diff --git a/arch/riscv/net/bpf_jit.h b/arch/riscv/net/bpf_jit.h
index f4b6b3b9edda..259294bdbc3a 100644
--- a/arch/riscv/net/bpf_jit.h
+++ b/arch/riscv/net/bpf_jit.h
@@ -15,7 +15,9 @@
 
 static inline bool rvc_enabled(void)
 {
-	return IS_ENABLED(CONFIG_RISCV_ISA_C);
+	return IS_ENABLED(CONFIG_PLATFORM_SUPPORTS_RISCV_ISA_C) ||
+		(IS_ENABLED(CONFIG_PLATFORM_MAY_SUPPORT_RISCV_ISA_C) &&
+			riscv_has_extension_likely(RISCV_ISA_EXT_c));
 }
 
 static inline bool rvzbb_enabled(void)

-- 
2.44.0