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Subject[PATCH v2 8/8] riscv: Add PLATFORM_SUPPORTS_RISCV_ISA_ZBS Kconfig option
Zbs can optimize kernel instruction sequences. Add a config option
PLATFORM_SUPPORTS_RISCV_ISA_ZBS that allows arbitrary Zbs instruction
sequences to be emitted by the compiler.

The existing "RISCV_ISA_ZBS" option is repurposed to be used to by kernel
code to determine if either PLATFORM_MAY_SUPPORT_RISCV_ISA_ZBS or
PLATFORM_SUPPORTS_RISCV_ISA_ZBS has been set.

Signed-off-by: Charlie Jenkins <charlie@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 b7399f236bba..60ae1bf71c70 100644
--- a/arch/riscv/Kconfig.isa
+++ b/arch/riscv/Kconfig.isa
@@ -340,3 +340,54 @@ config PLATFORM_SUPPORTS_RISCV_ISA_ZBC
systems that do not support the Zbc extension.

endchoice
+
+config TOOLCHAIN_HAS_ZBS
+ bool
+ default y
+ depends on !64BIT || $(cc-option,-mabi=lp64 -march=rv64ima_zbs)
+ depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zbs)
+ depends on LLD_VERSION >= 150000 || LD_VERSION >= 23900
+ depends on AS_HAS_OPTION_ARCH
+
+config RISCV_ISA_ZBS
+ bool
+
+choice
+ prompt "Zbs extension for bit manipulation instructions support"
+ default PLATFORM_MAY_SUPPORT_RISCV_ISA_ZBS
+ help
+ This selects the level of support for Zbs instructions to be
+ built into the Linux Kernel. This does not impact whether Zbs
+ instructions are allowed to be emitted by user-space code.
+
+ The Zbs extension provides instructions to accelerate carry-less
+ multiplication.
+
+config PROHIBIT_RISCV_ISA_ZBS
+ bool "Prohibit Zbs instruction sequences"
+ depends on NONPORTABLE
+ help
+ Regardless of if the platform supports Zbs instructions,
+ prohibit the kernel from emitting Zbs instructions.
+
+config PLATFORM_MAY_SUPPORT_RISCV_ISA_ZBS
+ bool "Allow Zbs instruction sequences if supported"
+ depends on TOOLCHAIN_HAS_ZBS
+ depends on RISCV_ALTERNATIVE
+ select RISCV_ISA_ZBS
+ help
+ Add support for enabling optimisations in the kernel when the
+ Zbs extension is detected at boot.
+
+config PLATFORM_SUPPORTS_RISCV_ISA_ZBS
+ bool "Emit Zbs instructions when building Linux"
+ depends on TOOLCHAIN_HAS_ZBS
+ depends on NONPORTABLE
+ select RISCV_ISA_ZBS
+ help
+ Adds "zbs" to the ISA subsets that the toolchain is allowed to emit
+ when building Linux, which results in Zbs instructions in the
+ Linux binary. This option produces a kernel that will not run on
+ systems that do not support the Zbs extension.
+
+endchoice
diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
index 57457d15e9a4..80ff8503196a 100644
--- a/arch/riscv/Makefile
+++ b/arch/riscv/Makefile
@@ -69,6 +69,7 @@ riscv-march-$(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) := $(riscv-march-y)_zbc
+riscv-march-$(CONFIG_PLATFORM_SUPPORTS_RISCV_ISA_ZBS) := $(riscv-march-y)_zbs

ifdef CONFIG_TOOLCHAIN_NEEDS_OLD_ISA_SPEC
KBUILD_CFLAGS += -Wa,-misa-spec=2.2
--
2.44.0


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