Re: [PATCH -next v14 19/19] riscv: Enable Vector code to be built

[Darius Rad <darius@bluespec.com>]

On Mon, Feb 27, 2023 at 10:18:34AM +0000, Conor Dooley wrote:
> Hey Andy,
> 
> On Fri, Feb 24, 2023 at 05:01:18PM +0000, Andy Chiu wrote:
> > From: Guo Ren <guoren@linux.alibaba.com>
> > 
> > This patch adds a config which enables vector feature from the kernel
> > space.
> > 
> > Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> > Co-developed-by: Greentime Hu <greentime.hu@sifive.com>
> > Signed-off-by: Greentime Hu <greentime.hu@sifive.com>
> > Suggested-by: Vineet Gupta <vineetg@rivosinc.com>
> > Suggested-by: Atish Patra <atishp@atishpatra.org>
> > Signed-off-by: Andy Chiu <andy.chiu@sifive.com>
> 
> At this point, you've basically re-written this patch and should be
> listed as a co-author at the very least!
> 
> > ---
> >  arch/riscv/Kconfig  | 18 ++++++++++++++++++
> >  arch/riscv/Makefile |  3 ++-
> >  2 files changed, 20 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index 81eb031887d2..19deeb3bb36b 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -418,6 +418,24 @@ config RISCV_ISA_SVPBMT
> >  
> >  	   If you don't know what to do here, say Y.
> >  
> > +config TOOLCHAIN_HAS_V
> > +	bool
> > +	default y
> > +	depends on !64BIT || $(cc-option,-mabi=lp64 -march=rv64iv)
> > +	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32iv)
> > +	depends on LLD_VERSION >= 140000 || LD_VERSION >= 23800
> > +
> > +config RISCV_ISA_V
> > +	bool "VECTOR extension support"
> > +	depends on TOOLCHAIN_HAS_V
> > +	select DYNAMIC_SIGFRAME
> 
> So, nothing here makes V depend on CONFIG_FPU...
> 
> > +	default y
> > +	help
> > +	  Say N here if you want to disable all vector related procedure
> > +	  in the kernel.
> > +
> > +	  If you don't know what to do here, say Y.
> > +
> >  config TOOLCHAIN_HAS_ZBB
> >  	bool
> >  	default y
> > diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> > index 76989561566b..375a048b11cb 100644
> > --- a/arch/riscv/Makefile
> > +++ b/arch/riscv/Makefile
> > @@ -56,6 +56,7 @@ riscv-march-$(CONFIG_ARCH_RV32I)	:= rv32ima
> >  riscv-march-$(CONFIG_ARCH_RV64I)	:= rv64ima
> >  riscv-march-$(CONFIG_FPU)		:= $(riscv-march-y)fd
> 
> ...but march only contains fd if CONFIG_FPU is enabled...
> 
> >  riscv-march-$(CONFIG_RISCV_ISA_C)	:= $(riscv-march-y)c
> > +riscv-march-$(CONFIG_RISCV_ISA_V)	:= $(riscv-march-y)v
> >  
> >  # Newer binutils versions default to ISA spec version 20191213 which moves some
> >  # instructions from the I extension to the Zicsr and Zifencei extensions.
> > @@ -65,7 +66,7 @@ riscv-march-$(toolchain-need-zicsr-zifencei) := $(riscv-march-y)_zicsr_zifencei
> >  # Check if the toolchain supports Zihintpause extension
> >  riscv-march-$(CONFIG_TOOLCHAIN_HAS_ZIHINTPAUSE) := $(riscv-march-y)_zihintpause
> >  
> > -KBUILD_CFLAGS += -march=$(subst fd,,$(riscv-march-y))
> > +KBUILD_CFLAGS += -march=$(subst fdv,,$(riscv-march-y))
> 
> ...so I think this will not work if !CONFIG_FPU && RISCV_ISA_V.
> IIRC, vector uses some floating point opcodes, but does it (or Linux's
> implementation) actually depend on having floating point support in the
> kernel?

Yes.

"The V extension requires the scalar processor implements the F and D
extensions", RISC-V "V" Vector Extension, Section 18.3. V: Vector Extension
for Application Processors.

> If not, this cannot be done in a oneliner. Otherwise, CONFIG_RISCV_ISA_V
> should explicitly depend on CONFIG_FPU.
> 
> >  KBUILD_AFLAGS += -march=$(riscv-march-y)
> >  
> >  KBUILD_CFLAGS += -mno-save-restore
> > -- 
> > 2.17.1
> > 
> >