Re: [PATCH v2] arch: arm64: add early_param idle=<wfi|yield|nop>

[Sudeep Holla <sudeep.holla@kernel.org>]

On Sat, Jul 11, 2026 at 09:35:25AM +0200, Yureka Lilian wrote:
> 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 <will@kernel.org>
> Signed-off-by: Yureka Lilian <yureka@cyberchaos.dev>
> ---
> Changes in v2:
> - Applied suggestions by Anshuman Khandual (Thanks!)
> - Link to v1: https://patch.msgid.link/20260705-arm64-idle-param-v1-1-7454249f473f@cyberchaos.dev
> ---
>  Documentation/admin-guide/kernel-parameters.txt | 23 +++++++++++++++++++
>  arch/arm64/kernel/idle.c                        | 30 +++++++++++++++++++++++--
>  arch/arm64/kernel/idle.h                        | 13 +++++++++++
>  arch/arm64/lib/delay.c                          |  5 ++++-
>  4 files changed, 68 insertions(+), 3 deletions(-)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index b2d7d3540ded..d7f5471edf8f 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2239,6 +2239,29 @@ 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.

Just curious as when and why one would need to use idle=wfi if that is the
default behaviour. I am missing the need to have it.

> +
> +			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: <bool>
>  			Allow force disabling of Shared Virtual Memory (SVA)
> diff --git a/arch/arm64/kernel/idle.c b/arch/arm64/kernel/idle.c
> index 05cfb347ec26..f161711a9954 100644
> --- a/arch/arm64/kernel/idle.c
> +++ b/arch/arm64/kernel/idle.c
> @@ -11,6 +11,27 @@
>  #include <asm/cpufeature.h>
>  #include <asm/sysreg.h>
>  
> +#include "idle.h"
> +
> +enum arm64_idle_mode 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 +47,13 @@ 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);


If WFI is replaced by NOP or YIELD, do we really need to save/restore
IRQ context used for pseudo-NMIs which may add some overhead ?

-- 
Regards,
Sudeep