Re: [PATCH v2] xen/riscv: add H extenstion to -march

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 3/24/25 1:31 PM, Jan Beulich wrote:

> On 21.03.2025 17:17, Oleksii Kurochko wrote:
> > H provides additional instructions and CSRs that control the new stage of
> > address translation and support hosting a guest OS in virtual S-mode
> > (VS-mode).
> >
> > According to the Unprivileged Architecture (version 20240411) specification:
> > ```
> > Table 74 summarizes the standardized extension names. The table also defines
> > the canonical order in which extension names must appear in the name string,
> > with top-to-bottom in table indicating first-to-last in the name string, e.g.,
> > RV32IMACV is legal, whereas RV32IMAVC is not.
> > ```
> > According to Table 74, the h extension is placed last in the one-letter
> > extensions name part of the ISA string.
> >
> > `h` is a standalone extension based on the patch [1] but it wasn't so
> > before.
> > As the minimal supported GCC version to build Xen for RISC-V is 12.2.0,
> > and for that version it will be needed to encode H extensions instructions
> > explicitly by checking if __risv_h is defined.
> >       
> Leaving aside the typo, what is this about? There's no use of __riscv_h in
> the patch here, and ...
>     
It is going to be used in future patches: https://gitlab.com/xen-project/people/olkur/xen/-/blob/riscv-next-upstreaming/xen/arch/riscv/p2m.c?ref_type=heads#L32
> > @@ -25,10 +24,13 @@ $(eval $(1) := \
> >  	$(call as-insn,$(CC) $(riscv-generic-flags)_$(1),$(value $(1)-insn),_$(1)))
> >  endef
> >  
> > +h-insn := "hfence.gvma"
> > +$(call check-extension,h)
> >       
> ... this, if it fails, will not have any effect on the build right now
> afaics.
>     
No, it won't have any affect now as instruction from H extension isn't used now.
But it will be needed for https://lore.kernel.org/xen-devel/dae753618491b2a6e42f7ed3f24190d0dc13fe3f.1740754166.git.Slavisa.Petrovic@xxxxxxxxx/
and for p2m changes mentioned above.

~ Oleksii