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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 22.09.2026 16:26, Oleksii Kurochko wrote:
> 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:

Quite a bit, yes; just one thing ...

> required_extensions[] panics at boot if Zihintpause is missing, but Xen 
> does not strictly require hardware support for it.

... here: required_extensions[] isn't a function and hence cannot "panic".

Jan

> 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