[PATCH v2 7/8] riscv: Add PLATFORM_SUPPORTS_RISCV_ISA_ZBC Kconfig option

[Charlie Jenkins]

Zbc can optimize kernel instruction sequences. Add a config option
PLATFORM_SUPPORTS_RISCV_ISA_ZBC that allows arbitrary Zbc instruction
sequences to be emitted by the compiler.

The existing "RISCV_ISA_ZBC" option is repurposed to be used to by kernel
code to determine if either PLATFORM_MAY_SUPPORT_RISCV_ISA_ZBC or
PLATFORM_SUPPORTS_RISCV_ISA_ZBC has been set.

Signed-off-by: Charlie Jenkins <charlie at rivosinc.com>
---
 arch/riscv/Kconfig.isa | 51 ++++++++++++++++++++++++++++++++++++++++++++++++++
 arch/riscv/Makefile    |  1 +
 2 files changed, 52 insertions(+)

diff --git a/arch/riscv/Kconfig.isa b/arch/riscv/Kconfig.isa
index e7f28dc44137..b7399f236bba 100644
--- a/arch/riscv/Kconfig.isa
+++ b/arch/riscv/Kconfig.isa
@@ -289,3 +289,54 @@ config PLATFORM_SUPPORTS_RISCV_ISA_ZBB
 	  systems that do not support the Zbb extension.
 
 endchoice
+
+config TOOLCHAIN_HAS_ZBC
+	bool
+	default y
+	depends on !64BIT || $(cc-option,-mabi=lp64 -march=rv64ima_zbc)
+	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zbc)
+	depends on LLD_VERSION >= 150000 || LD_VERSION >= 23900
+	depends on AS_HAS_OPTION_ARCH
+
+config RISCV_ISA_ZBC
+	bool
+
+choice
+	prompt "Zbc extension for bit manipulation instructions support"
+	default PLATFORM_MAY_SUPPORT_RISCV_ISA_ZBC
+	help
+	  This selects the level of support for Zbc instructions to be
+	  built into the Linux Kernel. This does not impact whether Zbc
+	  instructions are allowed to be emitted by user-space code.
+
+	  The Zbc extension provides instructions to accelerate carry-less
+	  multiplication.
+
+config PROHIBIT_RISCV_ISA_ZBC
+	bool "Prohibit Zbc instruction sequences"
+	depends on NONPORTABLE
+	help
+	  Regardless of if the platform supports Zbc instructions,
+	  prohibit the kernel from emitting Zbc instructions.
+
+config PLATFORM_MAY_SUPPORT_RISCV_ISA_ZBC
+	bool "Allow Zbc instruction sequences if supported"
+	depends on TOOLCHAIN_HAS_ZBC
+	depends on RISCV_ALTERNATIVE
+	select RISCV_ISA_ZBC
+	help
+	  Add support for enabling optimisations in the kernel when the
+	  Zbc extension is detected at boot.
+
+config PLATFORM_SUPPORTS_RISCV_ISA_ZBC
+	bool "Emit Zbc instructions when building Linux"
+	depends on TOOLCHAIN_HAS_ZBC
+	depends on NONPORTABLE
+	select RISCV_ISA_ZBC
+	help
+	  Adds "zbc" to the ISA subsets that the toolchain is allowed to emit
+	  when building Linux, which results in Zbc instructions in the
+	  Linux binary. This option produces a kernel that will not run on
+	  systems that do not support the Zbc extension.
+
+endchoice
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index c8ec38b9880a..57457d15e9a4 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -68,6 +68,7 @@ riscv-march-$(CONFIG_FPU)		:= $(riscv-march-y)fd
 riscv-march-$(CONFIG_PLATFORM_SUPPORTS_RISCV_ISA_C)	:= $(riscv-march-y)c
 riscv-march-$(CONFIG_PLATFORM_SUPPORTS_RISCV_ISA_ZBA)	:= $(riscv-march-y)_zba
 riscv-march-$(CONFIG_PLATFORM_SUPPORTS_RISCV_ISA_ZBB)	:= $(riscv-march-y)_zbb
+riscv-march-$(CONFIG_PLATFORM_SUPPORTS_RISCV_ISA_ZBC)	:= $(riscv-march-y)_zbc
 
 ifdef CONFIG_TOOLCHAIN_NEEDS_OLD_ISA_SPEC
 KBUILD_CFLAGS += -Wa,-misa-spec=2.2

-- 
2.44.0