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

[Yureka Lilian]

On 7/13/26 11:57, Sudeep Holla wrote:
> 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@xxxxxxxxxx/
> > Suggested-by: Will Deacon <will@xxxxxxxxxx>
> > Signed-off-by: Yureka Lilian <yureka@xxxxxxxxxxxxxx>
> > ---
> > Changes in v2:
> > - Applied suggestions by Anshuman Khandual (Thanks!)
> > - Link to v1: https://patch.msgid.link/20260705-arm64-idle-param-v1-1-7454249f473f@xxxxxxxxxxxxxx
> > ---
> >   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 ?

There are optimizations, even in the ARM64_IDLE_WFI case, which could be 
done here, such as checking that an interrupt actually occurred before 
continuing (and repeating the wfi/yield/nop until this is the case). I 
would prefer not to do these optimizations in this patch series, and 
leave it as future work, because I don't understand all the implications 
at this point. Is this acceptable for you?


Thanks,

— Yureka