Re: [PATCH] riscv: avoid enabling vectorized code generation

From: Palmer Dabbelt
Date: Wed Feb 08 2023 - 12:56:13 EST


On Fri, 16 Dec 2022 10:50:12 PST (-0800), abdulras@xxxxxxxxxx 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@xxxxxxxxxx>
---
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 += $(call cc-option,-mno-implicit-float)
+
ifeq ($(CONFIG_STACKPROTECTOR_PER_TASK),y)
prepare: stack_protector_prepare
stack_protector_prepare: prepare0

Sorry to just restart the thread, but there's been discussions on this in a bunch of places. From my understanding, we don't actually need this: we have this tricky line in the Makefile

KBUILD_CFLAGS += -march=$(subst fd,,$(riscv-march-y))

that removes the floating-point extenions from what the kernel is built with, so adding `-mno-implicit-float` doesn't actually do anything (and we'd end up with essentially the same thing for V when it gets added).

So unless I'm missing something, we don't need this.