Re: [PATCH 1/5] riscv: Checksum header

[Palmer Dabbelt]

On Sat, 26 Aug 2023 18:42:41 PDT (-0700), Conor Dooley wrote:
> On Sat, Aug 26, 2023 at 06:26:06PM -0700, Charlie Jenkins wrote:
> > Provide checksum algorithms that have been designed to leverage riscv
> > instructions such as rotate. In 64-bit, can take advantage of the larger
> > register to avoid some overflow checking.
> >
> > Add configuration for Zba extension and add march for Zba and Zbb.
> >
> > Signed-off-by: Charlie Jenkins <charlie@xxxxxxxxxxxx>
> > ---
> >  arch/riscv/Kconfig                | 23 +++++++++++
> >  arch/riscv/Makefile               |  2 +
> >  arch/riscv/include/asm/checksum.h | 86 +++++++++++++++++++++++++++++++++++++++
> >  3 files changed, 111 insertions(+)
> >
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index 4c07b9189c86..8d7e475ca28d 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -507,6 +507,29 @@ config RISCV_ISA_V_DEFAULT_ENABLE
> >  
> >  	  If you don't know what to do here, say Y.
> >  
> > +config TOOLCHAIN_HAS_ZBA
> > +	bool
> > +	default y
> > +	depends on !64BIT || $(cc-option,-mabi=lp64 -march=rv64ima_zba)
> > +	depends on !32BIT || $(cc-option,-mabi=ilp32 -march=rv32ima_zba)
> > +	depends on LLD_VERSION >= 150000 || LD_VERSION >= 23900
> > +	depends on AS_HAS_OPTION_ARCH
> > +
> > +config RISCV_ISA_ZBA
> > +	bool "Zba extension support for bit manipulation instructions"
> > +	depends on TOOLCHAIN_HAS_ZBA
> > +	depends on MMU
> > +	depends on RISCV_ALTERNATIVE
> > +	default y
> > +	help
> > +	   Adds support to dynamically detect the presence of the ZBA
> > +	   extension (basic bit manipulation) and enable its usage.
> > +
> > +	   The Zba extension provides instructions to accelerate a number
> > +	   of bit-specific address creation operations.
> > +
> > +	   If you don't know what to do here, say Y.
> > +
> >  config TOOLCHAIN_HAS_ZBB
> >  	bool
> >  	default y
> > diff --git a/arch/riscv/Makefile b/arch/riscv/Makefile
> > index 6ec6d52a4180..51fa3f67fc9a 100644
> > --- a/arch/riscv/Makefile
> > +++ b/arch/riscv/Makefile
> > @@ -61,6 +61,8 @@ riscv-march-$(CONFIG_ARCH_RV64I)	:= rv64ima
> >  riscv-march-$(CONFIG_FPU)		:= $(riscv-march-y)fd
> >  riscv-march-$(CONFIG_RISCV_ISA_C)	:= $(riscv-march-y)c
> >  riscv-march-$(CONFIG_RISCV_ISA_V)	:= $(riscv-march-y)v
> > +riscv-march-$(CONFIG_RISCV_ISA_ZBA)	:= $(riscv-march-y)_zba
> > +riscv-march-$(CONFIG_RISCV_ISA_ZBB)	:= $(riscv-march-y)_zbb
>
> AFAICT, this is going to break immediately on any system that enables
> RISCV_ISA_ZBA (which will happen by default) but does not support the
> extension. You made the option depend on RISCV_ALTERNATIVE, but I do
> not see any use of alternatives in the code to actually perform the
> dynamic detection of Zba.

I guess we kind of have an ambiguity here: for stuff like C we just 
unconditionally use the instructions, but for the rest we probe first.  
We should probably have three states for each extension: disabled, 
dynamically detected, and assumed.

> Note that for fd & v, we add it to riscv-march-y, but then immediately
> remove it again before passing to the compiler, only allow them in
> AFLAGS:
> 	# Remove F,D,V from isa string for all. Keep extensions between "fd" and "v" by
> 	# matching non-v and non-multi-letter extensions out with the filter ([^v_]*)
> 	KBUILD_CFLAGS += -march=$(shell echo $(riscv-march-y) | sed -E 's/(rv32ima|rv64ima)fd([^v_]*)v?/\1\2/')
>
> What am I missing?

FD and V both have state that can be saved lazily, so we can't let 
arbitrary code use them.  The extensions formally known as B don't add 
state, so they are safe to flip on in arbitrary places (aside from the 
issues you pointed out above).

> Thanks,
> Conor.