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

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

[Yureka Lilian]

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.
+
+			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);
 }
diff --git a/arch/arm64/kernel/idle.h b/arch/arm64/kernel/idle.h
new file mode 100644
index 000000000000..693f981c9a91
--- /dev/null
+++ b/arch/arm64/kernel/idle.h
@@ -0,0 +1,13 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+#ifndef __ARM64_KERNEL_IDLE_H
+#define __ARM64_KERNEL_IDLE_H
+
+extern enum arm64_idle_mode idle;
+
+enum arm64_idle_mode {
+	ARM64_IDLE_WFI,
+	ARM64_IDLE_YIELD,
+	ARM64_IDLE_NOP,
+};
+
+#endif
diff --git a/arch/arm64/lib/delay.c b/arch/arm64/lib/delay.c
index e278e060e78a..2452990ed37a 100644
--- a/arch/arm64/lib/delay.c
+++ b/arch/arm64/lib/delay.c
@@ -15,6 +15,8 @@
 
 #include <clocksource/arm_arch_timer.h>
 
+#include "../kernel/idle.h"
+
 #define USECS_TO_CYCLES(time_usecs)			\
 	xloops_to_cycles((time_usecs) * 0x10C7UL)
 
@@ -49,7 +51,8 @@ void __delay(unsigned long cycles)
 		 * Start with WFIT. If an interrupt makes us resume
 		 * early, use a WFET loop to complete the delay.
 		 */
-		wfit(end);
+		if (likely(idle == ARM64_IDLE_WFI))
+			wfit(end);
 		while ((__delay_cycles() - start) < cycles)
 			wfet(end);
 	} else 	if (arch_timer_evtstrm_available()) {

---
base-commit: bee763d5f341b99cf472afeb508d4988f62a6ca1
change-id: 20260705-arm64-idle-param-c27fc0e7ea05

Best regards,
--  
Yureka Lilian <yureka@cyberchaos.dev>

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

[Sudeep Holla]

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

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

[Will Deacon]

On Mon, Jul 13, 2026 at 10:57:38AM +0100, 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@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.

It's probably useful to have so that you can override an idle= option
present earlier in the cmdline. e.g.

	idle=nop idle=wfi

will give you the wfi behaviour, which is handy if the cmdline is
stitched topgether from different sources.

Will

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

[Sudeep Holla]

On Mon, Jul 13, 2026 at 12:59:31PM +0100, Will Deacon wrote:
> On Mon, Jul 13, 2026 at 10:57:38AM +0100, 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@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.
> 
> It's probably useful to have so that you can override an idle= option
> present earlier in the cmdline. e.g.
> 
> 	idle=nop idle=wfi
> 
> will give you the wfi behaviour, which is handy if the cmdline is
> stitched topgether from different sources.
> 

Thanks, that makes sense.

-- 
Regards,
Sudeep

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

[Anshuman Khandual]

On 13/07/26 3:27 PM, 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@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.

I guess once there is a list to chose options from
in the command line, should not the default option 
be listed there as well ?

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

[Sudeep Holla]

On Mon, Jul 13, 2026 at 05:54:16PM +0530, Anshuman Khandual wrote:
> On 13/07/26 3:27 PM, 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@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.
> 
> I guess once there is a list to chose options from in the command line,
> should not the default option be listed there as well ?
> 

I don't know if it is a requirement or just a practice. But for reasons
Will mentioned, it becomes a requirement.

-- 
Regards,
Sudeep