From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id EF58A3E2AB6; Mon, 13 Jul 2026 09:57:45 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783936668; cv=none; b=hWqM4hKU+ZxvyjoZ6MrK44l6smJ42mmUQk/3zacwJmZmk7Pst0HLK0JSfdf/Ueg1HqdKhydH/82MH7LnEMbjwnsG6iDuF+P7j/W5w7diHIsljSUIMCOXCyGm1uAKjCMPl29nMmXuoYQ8IPW1XyNJHYGKElB1x9d3IFPSST31r6A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783936668; c=relaxed/simple; bh=la3YNnos8fO30PpKphG30Z8QyQ2Elt6Z+LYql8V1iyU=; h=Date:From:To:Cc:Subject:Message-ID:References:MIME-Version: Content-Type:Content-Disposition:In-Reply-To; b=kDBhxKZ+QywuVefpQ2sitsFz4zhyjGrfjA9iexurn8cCw3CjeOMVGDgLPs9B5aBUkVv5HA3GinNH5nVq5MKHgt9mJGBKfBcBe1R/8CxpP97AL0yZOFIjx7TwloXmbGBP7/r2ZjuiinEReFDSVbU/yR1FuIGzDcnWVl7V82sht7E= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=hfTueQg9; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="hfTueQg9" Received: by smtp.kernel.org (Postfix) with ESMTPSA id EAB261F00A3D; Mon, 13 Jul 2026 09:57:40 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1783936662; bh=aBrXVBLeI2IEHvIvaOWKwyqAdMHFb6oga++/AXMhGxQ=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=hfTueQg900K2ypOWxlKMzGOIEgWSbJrvzJnxo3vCHRq+jA7KVhy3HOtmzQ5/GWEEl lpyAc37NQF69qqd/vVEsM3k1mfO28ktyM9yxPIRjpREeBcMfMniJdMWENMEqn5taMN k4ZenW3aSYUU6w3LQswfS7bPvbr41Zd8gC4HPSW3fQvxOvsEfjetms3C5rSp8zLC4L bFF5zuM8pCjti4SLwujr2ti0l0Wo1NKFpbPj7bTodj8FCcZG4ZCtis+zBRNPZtislV RS+8xBcr9nv0x9BOtuPHH1L1IrbTHSN3whtbtue3Dhc1o10q/sl0R/oeKDWJHa3omJ OoloLUPtEuoQA== Date: Mon, 13 Jul 2026 10:57:38 +0100 From: Sudeep Holla To: Yureka Lilian Cc: Jonathan Corbet , Sudeep Holla , Shuah Khan , Catalin Marinas , Will Deacon , Anshuman Khandual , linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org Subject: Re: [PATCH v2] arch: arm64: add early_param idle= Message-ID: <20260713-bulky-thistle-leopard-c66a18@sudeepholla> References: <20260711-arm64-idle-param-v2-1-0ab67652a435@cyberchaos.dev> Precedence: bulk X-Mailing-List: linux-doc@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260711-arm64-idle-param-v2-1-0ab67652a435@cyberchaos.dev> 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 > Signed-off-by: Yureka Lilian > --- > 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: > 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 > #include > > +#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