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

[Oleksii Kurochko <oleksii.kurochko@gmail.com>]

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@vates.tech>
>> ---
>> 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