note that Event Stream periodically generates wakeup event for WFE, linux kernel enables it and configures it to 100Hz (10us). https://developer.arm.com/documentation/102379/0104/The-processor-timers/Event-stream
saeed On Sat, Jun 20, 2026 at 8:46 PM Wathsala Vithanage <[email protected]> wrote: > > > On 6/18/26 01:17, Hemant Agrawal wrote: > > Hi Watshala, > > I think WFET is not available on A72 core. > > Can you update your answer w.r.t Cortex-A72/Arm v8.0 architecture? > > WFET is not available on A72. We enabled it in DPDK selectively for this > reason. > > > > > Regards > > Hemant > > > > > >> -----Original Message----- > >> From: Wathsala Vithanage <[email protected]> > >> Sent: 17 June 2026 17:27 > >> To: Hemant Agrawal <[email protected]>; Morten Brørup > >> <[email protected]> > >> Cc: [email protected]; Maxime Leroy <[email protected]>; Gagandeep Singh > >> <[email protected]> > >> Subject: Re: ARM v8 rte_power_pause > >> Importance: High > >> > >> Hi Morten and Hemant, > >> > >> YIELD is a NOP on non-SMT CPUs, such as Neoverse. > >> > >> WFE is universally available on AArch64, but it comes with a caveat: the > >> CPU > >> can remain in a low-power state indefinitely unless an event is triggered. > >> That > >> event can be generated explicitly via SEV/SEVL by a different CPU, or > >> implicitly > >> through address monitoring (LDAXR). > >> > >> WFET is the safer variant because it includes a timeout, so explicit or > >> implicit > >> event-register manipulation is not required. > >> > >> --wathsala > >> > >> On 6/12/26 01:11, Hemant Agrawal wrote: > >>> Hi Morten, > >>> On Cortex‑A72 (ARMv8), the only architectural primitives available are > >> YIELD, WFE, and WFI: > >>> YIELD is the only deterministic, low-overhead option (pure CPU relax, > >> no entry into low-power state) > >>> WFE can be used as a low-power idle hint, but it is event-driven and > >> not time-based (it may return immediately) > >>> WFI depends on interrupt wakeup and is therefore not suitable for > >>> tight latency loops > >>> > >>> For ~1 µs latency targets, the practical approach is a hybrid strategy: > >>> > >>> Short waits → spin using YIELD > >>> Slightly longer waits → opportunistically use WFE for power reduction > >>> > >>> A simple implementation could look like (not tested): > >>> > >>> static inline void rte_armv8_pause(unsigned int iters) { > >>> if (iters < 64) { > >>> for (unsigned int i = 0; i < iters; i++) > >>> asm volatile("yield"); > >>> } else { > >>> asm volatile("sevl"); > >>> asm volatile("wfe"); > >>> } > >>> } > >>> > >>> @Wathsala Vithanage — would appreciate your thoughts, especially if there > >> are any micro-architectural nuances we should consider. > >>> Regards, > >>> Hemant > >>> > >>>> -----Original Message----- > >>>> From: Morten Brørup <[email protected]> > >>>> Sent: 03 June 2026 17:26 > >>>> To: Wathsala Vithanage <[email protected]>; Hemant Agrawal > >>>> <[email protected]>; Sachin Saxena (OSS) > >>>> <[email protected]> > >>>> Cc: [email protected]; Maxime Leroy <[email protected]> > >>>> Subject: ARM v8 rte_power_pause > >>>> Importance: High > >>>> > >>>> Hi Wathsala, Hemant and Sachin, > >>>> > >>>> Over at the Grout project, we are discussing power management in the > >>>> context of 100 Gbit/s latency deadlines [1]. > >>>> > >>>> rte_power_pause() is not implemented for ARM v8 / Cortex-A72. > >>>> Syscalls such as nanosleep() have too much overhead, and cannot be used. > >>>> > >>>> Any suggestions for a power-reducing method to make a CPU core "sleep" > >> (i.e. > >>>> do nothing) for durations in the order of 1 microsecond? > >>>> > >>>> [1]: > >>>> https://eur01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgit > >>>> > >> hu%2F&data=05%7C02%7Chemant.agrawal%40nxp.com%7C06a651571db > >> 545d47d7a0 > >> 8decc67908e%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0%7C0%7C639 > >> 172942353 > >> 967617%7CUnknown%7CTWFpbGZsb3d8eyJFbXB0eU1hcGkiOnRydWUsIlYi > >> OiIwLjAuMD > >> AwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTWFpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7 > >> C%7C&s > >> data=7NCh3%2BS3TAu1sRLYgqGNAaTwqdgwjqhAs2awPixIeEM%3D&reserve > >> d=0 > >>>> b.com%2FDPDK%2Fgrout%2Fpull%2F624%23issuecomment- > >>>> > >> 4602036364&data=05%7C02%7Chemant.agrawal%40nxp.com%7Cdbff5f2e > >> 8db1406f0c4008dec1671791%7C686ea1d3bc2b4c6fa92cd99c5c301635%7 > >> C0%7C0%7C639160845728472826%7CUnknown%7CTWFpbGZsb3d8eyJFb > >> XB0eU1hcGkiOnRydWUsIlYiOiIwLjAuMDAwMCIsIlAiOiJXaW4zMiIsIkFOIjoiTW > >> FpbCIsIldUIjoyfQ%3D%3D%7C0%7C%7C%7C&sdata=DRpJWjm2yaF3Cnhk0b > >>>> bFFhmGbKRweOOiWdsWco2NbX0%3D&reserved=0 > >>>> > >>>> -Morten
