Re: [PATCH v7 03/19] xen/riscv: introduce extenstion support check by compiler

[Oleksii <oleksii.kurochko@xxxxxxxxx>]

On Fri, 2024-04-05 at 08:08 +0200, Jan Beulich wrote:
> On 04.04.2024 18:17, Oleksii wrote:
> > On Thu, 2024-04-04 at 17:43 +0200, Jan Beulich wrote:
> > > On 04.04.2024 17:18, Oleksii wrote:
> > > > On Thu, 2024-04-04 at 12:07 +0200, Jan Beulich wrote:
> > > > > On 03.04.2024 12:19, Oleksii Kurochko wrote:
> > > > > > --- a/xen/arch/riscv/arch.mk
> > > > > > +++ b/xen/arch/riscv/arch.mk
> > > > > > @@ -3,16 +3,27 @@
> > > > > >  
> > > > > >  $(call cc-options-add,CFLAGS,CC,$(EMBEDDED_EXTRA_CFLAGS))
> > > > > >  
> > > > > > -CFLAGS-$(CONFIG_RISCV_64) += -mabi=lp64
> > > > > > +riscv-abi-$(CONFIG_RISCV_32) := -mabi=ilp32
> > > > > > +riscv-abi-$(CONFIG_RISCV_64) := -mabi=lp64
> > > > > >  
> > > > > >  riscv-march-$(CONFIG_RISCV_ISA_RV64G) := rv64g
> > > > > >  riscv-march-$(CONFIG_RISCV_ISA_C)       := $(riscv-march-
> > > > > > y)c
> > > > > >  
> > > > > > +riscv-generic-flags := $(riscv-abi-y) -march=$(riscv-
> > > > > > march-y)
> > > > > > +
> > > > > > +zbb := $(call as-insn,$(CC) $(riscv-generic-
> > > > > > flags)_zbb,"",_zbb)
> > > > > 
> > > > > While committing another question popped up: Why "" (i.e. no
> > > > > insn)
> > > > > here, ...
> > > > > 
> > > > > > +zihintpause := $(call as-insn,\
> > > > > > +               $(CC) $(riscv-generic-
> > > > > > flags)_zihintpause,"pause",_zihintpause)
> > > > > 
> > > > > ... but "pause" here?
> > > > 
> > > > In the case of the Zbb extension, we don't check for a specific
> > > > instruction, but with the Zihintpause, the idea was to verify
> > > > if
> > > > the
> > > > pause instruction is supported or not.
> > > 
> > > And why's this verification relevant here, but not for Zbb?
> > It is not relevant and can be dropped.
> 
> Is it really not? Aren't you checking two things for Zihintpause
> (compiler
> and assembler support), while checking only one (compiler) for Zbb?
You are right.
I made again an assumption that binutils are as new as a compiler.

Then it makes sense to replace an argument if as-insn "" with andn t0,
t0, t0 for Zbb:

   -zbb := $(call as-insn,$(CC) $(riscv-generic-flags)_zbb,"",_zbb)
   +zbb_insn := "andn t0, t0, t0"
   +zbb := $(call as-insn,$(CC) $(riscv-generic-
   flags)_zbb,${zbb_insn},_zbb)

~ Oleksii

> 
> Jan