Re: [PATCH v2 4/6] xen/riscv: make Zihintpause no longer a required extension

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 9/22/26 2:27 PM, Jan Beulich wrote:
> On 10.09.2026 11:34, Baptiste Le Duc wrote:
> > required_extensions[] panics at boot if Zihintpause is missing, but Xen
> > never actually depends on it: cpu_relax() only emits the "pause" hint when
> > the extension is implemented, otherwise it emits `0x0100000F`, a legally
> > valid FENCE instruction (`FENCE W, 0`) rather than a native NOP.
> Just that PAUSE's encoding is 0x0100000F. I.e. what is emitted is always
> the same, and hence discussing the encoding aspect here doesn't help
> justify the change. NOP or not also doesn't really matter here. The
> specific hint encoding looks to fall into what prior to Zihintpause would
> have been covered by "Designated for future standard use", and hence ...
>
> > FENCE is
> > guaranteed by the RISC-V base ISA, so it never raises an illegal
> > instruction fault. With an empty successor set, it enforces no
> > memory-ordering constraints and thus architecturally behaves as a NOP.
> ... there indeed should be no concern for any platform playing by the
> rules.
>
> > Drop it from required_extensions so hardware without Zihintpause
> > still boots.
> >
> > Assisted-by: Claude:claude-opus-5
> > Signed-off-by: Baptiste Le Duc <baptiste.le-duc@xxxxxxxxxx>
> > ---
> > Changes since v1:
> > - rewrite commit message.
> I fear another round of re-writing is going to be necessary, sorry.
Would it be better:

required_extensions[] panics at boot if Zihintpause is missing, but Xen 
does not strictly require hardware support for it.
The PAUSE hint (Zihintpause extension) is encoded as FENCE W, 0 
(0x0100000F). In accordance with the RISC-V Unprivileged ISA, HINTs are 
encoded in the space of valid standard instructions. On platforms 
without Zihintpause support, executing 0x0100000F is treated as a 
standard FENCE with an empty successor set, which acts as a NOP and 
never generates an illegal instruction trap.
Therefore, Zihintpause is purely an optimization hint. Drop it from 
required_extensions[] so systems without explicit Zihintpause support 
can boot Xen successfully.
?

~ Oleksii