linux-kernel - Re: [PATCH] riscv: avoid enabling vectorized code generation

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Date:   Thu, 22 Dec 2022 00:17:07 +0800
From:   Bin Meng <bmeng.cn@...il.com>
To:     Saleem Abdulrasool <abdulras@...gle.com>
Cc:     Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] riscv: avoid enabling vectorized code generation

Hi,

On Sat, Dec 17, 2022 at 3:12 AM Saleem Abdulrasool <abdulras@...gle.com> wrote:
>
> The compiler is free to generate vectorized operations for zero'ing
> memory.  The kernel does not use the vector unit on RISCV, similar to
> architectures such as x86 where we use `-mno-mmx` et al to prevent the
> implicit vectorization.  Perform a similar check for
> `-mno-implicit-float` to avoid this on RISC-V targets.
>
> Signed-off-by: Saleem Abdulrasool <abdulras@...gle.com>
> ---
>  arch/riscv/Makefile | 4 ++++
>  1 file changed, 4 insertions(+)
>
> diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> index 0d13b597cb55..68433476a96e 100644
> --- a/arch/riscv/Makefile
> +++ b/arch/riscv/Makefile
> @@ -89,6 +89,10 @@ KBUILD_AFLAGS_MODULE += $(call as-option,-Wa$(comma)-mno-relax)
>  # architectures.  It's faster to have GCC emit only aligned accesses.
>  KBUILD_CFLAGS += $(call cc-option,-mstrict-align)
>
> +# Ensure that we do not vectorize the kernel code when the `v` extension is
> +# enabled.  This mirrors the `-mno-mmx` et al on x86.
> +KBUILD_CFLAGS += $(call cc-option,-mno-implicit-float)

This looks like an LLVM flag, but not GCC.

Can you elaborate what exact combination (compiler flag and source)
would cause an issue?

>From your description, I guess it's that when enabling V extension in
LLVM, the compiler tries to use vector instructions to zero memory,
correct?

Can you confirm LLVM does not emit any float instructions (like F/D
extensions) because the flag name suggests something like "float"?

> +
>  ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
>  prepare: stack_protector_prepare
>  stack_protector_prepare: prepare0
> --

Regards,
Bin