From d97afae6f16a4f8ac7d50af0070816270b5380c1 Mon Sep 17 00:00:00 2001 From: Yureka Lilian Date: Tue, 4 Aug 2026 19:23:15 +0200 Subject: [PATCH] arch: arm64: add early_param idle= Overriding the idle mechanism might be useful for debugging and performance testing. Add a cmdline parameter for it, similar to the existing idle= parameter already present for the x86 and ppc architectures. It is also useful on platforms where the WFI instruction misbehaves, such as Apple Silicon SoCs. Generally, a misbehaving instruction should be treated as an erratum and patched using the alternatives framework. However, in the Apple Silicon case we need more flexibility because it is difficult to detect whether the erratum applies. For example, Linux VMs inside macOS have the same MIDR and may even seem like they're running in EL2 in the case of NV, but should continue using WFI (it's trapped and handled correctly by the hypervisor there). Thus, we prefer to let the m1n1 bootloader add the idle=nop parameter[1]. Link[1]: https://lore.kernel.org/all/99b69262-e54b-424e-baa2-96ef7013b87a@kernel.org/ Suggested-by: Will Deacon Signed-off-by: Yureka Lilian Signed-off-by: Will Deacon --- .../admin-guide/kernel-parameters.txt | 23 +++++++++++++ arch/arm64/kernel/idle.c | 32 +++++++++++++++++-- 2 files changed, 53 insertions(+), 2 deletions(-) diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt index b5493a7f8f22..7e9cc49b90a9 100644 --- a/Documentation/admin-guide/kernel-parameters.txt +++ b/Documentation/admin-guide/kernel-parameters.txt @@ -2236,6 +2236,29 @@ Kernel parameters idle=nomwait: Disable mwait for CPU C-states + [ARM64,EARLY] + Format: idle=wfi, idle=yield, idle=nop + + idle=wfi: Use the WFI (Wait For Interrupt) hint + instruction in the idle loop. This is the default and + allows the CPU to enter a low-power state until an + interrupt arrives. + + idle=yield: Use the YIELD hint instruction instead of + WFI. CPUs supporting simultaneous multi-threading (SMT), + can continue executing another thread when the current + thread reaches the idle loop. This will make the CPUs + eat more power, but may be useful to get slightly better + performance in some applications, since the CPUs will + not enter a low-power state. + + idle=nop: Do not execute any idle instruction in the + idle loop. This is useful on platforms where WFI + misbehaves, leading to system instability or loss of CPU + state. This will make the CPUs eat more power, but may + give slightly better performance in some applications, + since the CPUs will not enter a low-power state. + idxd.sva= [HW] Format: Allow force disabling of Shared Virtual Memory (SVA) diff --git a/arch/arm64/kernel/idle.c b/arch/arm64/kernel/idle.c index 05cfb347ec26..42c5543f9589 100644 --- a/arch/arm64/kernel/idle.c +++ b/arch/arm64/kernel/idle.c @@ -11,6 +11,29 @@ #include #include +enum { + ARM64_IDLE_WFI, + ARM64_IDLE_YIELD, + ARM64_IDLE_NOP, +} idle = ARM64_IDLE_WFI; + +static int __init setup_idle(char *arg) +{ + if (!arg) + return -1; + else if (!strcmp(arg, "wfi")) + idle = ARM64_IDLE_WFI; + else if (!strcmp(arg, "yield")) + idle = ARM64_IDLE_YIELD; + else if (!strcmp(arg, "nop")) + idle = ARM64_IDLE_NOP; + else + return -1; + + return 0; +} +early_param("idle", setup_idle); + /* * cpu_do_idle() * @@ -26,8 +49,13 @@ void __cpuidle cpu_do_idle(void) arm_cpuidle_save_irq_context(&context); - dsb(sy); - wfi(); + if (likely(idle == ARM64_IDLE_WFI)) { + dsb(sy); + wfi(); + } else if (idle == ARM64_IDLE_YIELD) { + dsb(sy); + asm volatile("yield" ::: "memory"); + } arm_cpuidle_restore_irq_context(&context); }