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SubjectRe: [PATCH v4 3/5] Cleanup ISA string setting
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On Tue, 07 Aug 2018 20:24:43 PDT (-0700), alankao@andestech.com wrote:
> Just a side note: (Assume that atomic and compressed is on)
>
> Before this patch, assembler was always given the riscv64imafdc
> MARCH string because there are fld/fsd's in entry.S; compiler was
> always given riscv64imac because kernel doesn't need floating point
> code generation. After this, the MARCH string in AFLAGS and CFLAGS
> become the same.
>
> Signed-off-by: Alan Kao <alankao@andestech.com>
> Cc: Greentime Hu <greentime@andestech.com>
> Cc: Vincent Chen <vincentc@andestech.com>
> Cc: Zong Li <zong@andestech.com>
> Cc: Nick Hu <nickhu@andestech.com>
> Reviewed-by: Christoph Hellwig <hch@lst.de>
> ---
> arch/riscv/Makefile | 19 ++++++++-----------
> 1 file changed, 8 insertions(+), 11 deletions(-)
>
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index 6d4a5f6c3f4f..e0fe6790624f 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -26,7 +26,6 @@ ifeq ($(CONFIG_ARCH_RV64I),y)
>
> KBUILD_CFLAGS += -mabi=lp64
> KBUILD_AFLAGS += -mabi=lp64
> - KBUILD_MARCH = rv64im
> LDFLAGS += -melf64lriscv
> else
> BITS := 32
> @@ -34,22 +33,20 @@ else
>
> KBUILD_CFLAGS += -mabi=ilp32
> KBUILD_AFLAGS += -mabi=ilp32
> - KBUILD_MARCH = rv32im
> LDFLAGS += -melf32lriscv
> endif
>
> KBUILD_CFLAGS += -Wall
>
> -ifeq ($(CONFIG_RISCV_ISA_A),y)
> - KBUILD_ARCH_A = a
> -endif
> -ifeq ($(CONFIG_RISCV_ISA_C),y)
> - KBUILD_ARCH_C = c
> -endif
> -
> -KBUILD_AFLAGS += -march=$(KBUILD_MARCH)$(KBUILD_ARCH_A)fd$(KBUILD_ARCH_C)
> +# ISA string setting
> +riscv-march-$(CONFIG_ARCH_RV32I) := rv32im
> +riscv-march-$(CONFIG_ARCH_RV64I) := rv64im
> +riscv-march-$(CONFIG_RISCV_ISA_A) := $(riscv-march-y)a
> +riscv-march-y := $(riscv-march-y)fd
> +riscv-march-$(CONFIG_RISCV_ISA_C) := $(riscv-march-y)c
> +KBUILD_CFLAGS += -march=$(riscv-march-y)
> +KBUILD_AFLAGS += -march=$(riscv-march-y)
>
> -KBUILD_CFLAGS += -march=$(KBUILD_MARCH)$(KBUILD_ARCH_A)$(KBUILD_ARCH_C)

We used to have "fd" disabled in KBUILD_CFLAGS because we wanted to ensure that
any use of floating-point types within the kernel would trigger the inclusion
of soft-float libraries rather than emitting hardware floating-point
instructions. The worry here is that we'd end up corrupting the user's
floating-point state with the kernel accesses because of the lazy save/restore
stuff going on.

I remember at some point it was illegal to use floating-point within the
kernel, but for some reason I thought that had changed. I do see a handful of
references to "float" and "double" in the kernel source, and most of references
to kernel_fpu_begin() appear to be in SSE-specific stuff. My one worry are the
usages in drivers/gpu/drm/amd/display/dc/calcs/dcn_calcs.c, as I can't quickly
demonstrate they won't happen.

If we do this I think we should at least ensure kernel_fpu_{begin,end}() do the
right thing for RISC-V. I'd still feel safer telling the C compiler to
disallow floating-point, though, as I'm a bit paranoid that GCC might go and
emit a floating-point instruction even when it's not explicitly asked for. I
just asked Jim, who actually understands GCC, and he said that it'll spill to
floating-point registers if the cost function determines it's cheaper than the
stack. While he thinks that's unlikely, I don't really want to rely a GCC cost
function for the correctness of our kernel port.

I'd like to avoid having "-march=*fd*" in CFLAGS, unless someone can convince
me it's safe and that I'm just being too paranoid :).

> KBUILD_CFLAGS += -mno-save-restore
> KBUILD_CFLAGS += -DCONFIG_PAGE_OFFSET=$(CONFIG_PAGE_OFFSET)

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