[PATCH v2 10/14] riscv: Add support for kernel-mode FPU

[Palmer Dabbelt]

On Wed, 27 Dec 2023 17:42:00 PST (-0800), samuel.holland at sifive.com wrote:
> This is motivated by the amdgpu DRM driver, which needs floating-point
> code to support recent hardware. That code is not performance-critical,
> so only provide a minimal non-preemptible implementation for now.
>
> Signed-off-by: Samuel Holland <samuel.holland at sifive.com>
> ---
>
> Changes in v2:
>  - Remove RISC-V architecture-specific preprocessor check
>
>  arch/riscv/Kconfig                  |  1 +
>  arch/riscv/Makefile                 |  3 +++
>  arch/riscv/include/asm/fpu.h        | 16 ++++++++++++++++
>  arch/riscv/kernel/Makefile          |  1 +
>  arch/riscv/kernel/kernel_mode_fpu.c | 28 ++++++++++++++++++++++++++++
>  5 files changed, 49 insertions(+)
>  create mode 100644 arch/riscv/include/asm/fpu.h
>  create mode 100644 arch/riscv/kernel/kernel_mode_fpu.c
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 24c1799e2ec4..4d4d1d64ce34 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -27,6 +27,7 @@ config RISCV
>  	select ARCH_HAS_GCOV_PROFILE_ALL
>  	select ARCH_HAS_GIGANTIC_PAGE
>  	select ARCH_HAS_KCOV
> +	select ARCH_HAS_KERNEL_FPU_SUPPORT if FPU
>  	select ARCH_HAS_MMIOWB
>  	select ARCH_HAS_NON_OVERLAPPING_ADDRESS_SPACE
>  	select ARCH_HAS_PMEM_API
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index a74be78678eb..2e719c369210 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -81,6 +81,9 @@ KBUILD_CFLAGS += -march=$(shell echo $(riscv-march-y) | sed -E 's/(rv32ima|rv64i
>
>  KBUILD_AFLAGS += -march=$(riscv-march-y)
>
> +# For C code built with floating-point support, exclude V but keep F and D.
> +CC_FLAGS_FPU  := -march=$(shell echo $(riscv-march-y) | sed -E 's/(rv32ima|rv64ima)([^v_]*)v?/\1\2/')
> +
>  KBUILD_CFLAGS += -mno-save-restore
>  KBUILD_CFLAGS += -DCONFIG_PAGE_OFFSET=$(CONFIG_PAGE_OFFSET)
>
> diff --git a/arch/riscv/include/asm/fpu.h b/arch/riscv/include/asm/fpu.h
> new file mode 100644
> index 000000000000..91c04c244e12
> --- /dev/null
> +++ b/arch/riscv/include/asm/fpu.h
> @@ -0,0 +1,16 @@
> +/* SPDX-License-Identifier: GPL-2.0-only */
> +/*
> + * Copyright (C) 2023 SiFive
> + */
> +
> +#ifndef _ASM_RISCV_FPU_H
> +#define _ASM_RISCV_FPU_H
> +
> +#include <asm/switch_to.h>
> +
> +#define kernel_fpu_available()	has_fpu()
> +
> +void kernel_fpu_begin(void);
> +void kernel_fpu_end(void);
> +
> +#endif /* ! _ASM_RISCV_FPU_H */
> diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
> index fee22a3d1b53..662c483e338d 100644
> --- a/arch/riscv/kernel/Makefile
> +++ b/arch/riscv/kernel/Makefile
> @@ -62,6 +62,7 @@ obj-$(CONFIG_MMU) += vdso.o vdso/
>
>  obj-$(CONFIG_RISCV_MISALIGNED)	+= traps_misaligned.o
>  obj-$(CONFIG_FPU)		+= fpu.o
> +obj-$(CONFIG_FPU)		+= kernel_mode_fpu.o
>  obj-$(CONFIG_RISCV_ISA_V)	+= vector.o
>  obj-$(CONFIG_SMP)		+= smpboot.o
>  obj-$(CONFIG_SMP)		+= smp.o
> diff --git a/arch/riscv/kernel/kernel_mode_fpu.c b/arch/riscv/kernel/kernel_mode_fpu.c
> new file mode 100644
> index 000000000000..0ac8348876c4
> --- /dev/null
> +++ b/arch/riscv/kernel/kernel_mode_fpu.c
> @@ -0,0 +1,28 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +/*
> + * Copyright (C) 2023 SiFive
> + */
> +
> +#include <linux/export.h>
> +#include <linux/preempt.h>
> +
> +#include <asm/csr.h>
> +#include <asm/fpu.h>
> +#include <asm/processor.h>
> +#include <asm/switch_to.h>
> +
> +void kernel_fpu_begin(void)
> +{
> +	preempt_disable();
> +	fstate_save(current, task_pt_regs(current));
> +	csr_set(CSR_SSTATUS, SR_FS);
> +}
> +EXPORT_SYMBOL_GPL(kernel_fpu_begin);
> +
> +void kernel_fpu_end(void)
> +{
> +	csr_clear(CSR_SSTATUS, SR_FS);
> +	fstate_restore(current, task_pt_regs(current));
> +	preempt_enable();
> +}
> +EXPORT_SYMBOL_GPL(kernel_fpu_end);

Reviewed-by: Palmer Dabbelt <palmer at rivosinc.com>
Acked-by: Palmer Dabbelt <palmer at rivosinc.com>

assuming you want to keep these together -- it touches a lot of stuff, 
so LMK if you want me to pick something up.

Thanks!