linux-kernel - Re: [PATCH v6 6/6] arm64: Use SYSTEM_OFF2 PSCI call to power off for hibernate

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Message-ID: <691447A1-8F3F-4890-B00F-8068A14CA126@infradead.org>
Date: Thu, 24 Oct 2024 17:56:09 +0200
From: David Woodhouse <dwmw2@...radead.org>
To: Oliver Upton <oliver.upton@...ux.dev>
CC: Miguel Luis <miguel.luis@...cle.com>, Paolo Bonzini <pbonzini@...hat.com>,
 Jonathan Corbet <corbet@....net>, Marc Zyngier <maz@...nel.org>,
 James Morse <james.morse@....com>, Suzuki K Poulose <suzuki.poulose@....com>,
 Zenghui Yu <yuzenghui@...wei.com>, Catalin Marinas <catalin.marinas@....com>,
 Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Lorenzo Pieralisi <lpieralisi@...nel.org>,
 "Rafael J. Wysocki" <rafael@...nel.org>, Pavel Machek <pavel@....cz>,
 Len Brown <len.brown@...el.com>, Shuah Khan <shuah@...nel.org>,
 David Woodhouse <dwmw@...zon.co.uk>,
 "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
 "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "linux-arm-kernel@...ts.infradead.org" <linux-arm-kernel@...ts.infradead.org>,
 "kvmarm@...ts.linux.dev" <kvmarm@...ts.linux.dev>,
 "linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
 "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
 Francesco Lavra <francescolavra.fl@...il.com>
Subject: Re: [PATCH v6 6/6] arm64: Use SYSTEM_OFF2 PSCI call to power off for hibernate

On 24 October 2024 17:44:49 CEST, Oliver Upton <oliver.upton@...ux.dev> wrote:
>Hi,
>
>On Thu, Oct 24, 2024 at 03:48:26PM +0200, David Woodhouse wrote:
>> On 24 October 2024 14:54:41 CEST, Miguel Luis <miguel.luis@...cle.com> wrote:
>> >Perhaps spec. F.b. could be accommodated by first invoking SYSTEM_OFF2 with
>> >PSCI_1_3_OFF_TYPE_HIBERNATE_OFF and checking its return value in case of a
>> >fallback to an invocation with 0x0 ?
>
>This already complies with F.b.
>
>The PSCI implementation is required to accept either 0 or 1 for
>HIBERNATE_OFF. Using 0 seems like a good choice for compatibility since ...
>
>> I wasn't aware there was any point. Are there any hypervisors which actually implemented it that way? Amazon Linux and Ubuntu guests already just use zero.
>> 
>> We could add it later if such a hypervisor (now in violation of F.b) turns up, I suppose?
>
>IIUC, you're really wanting to 0x0 because there are hypervisors out
>there that violate the final spec and *only* accept this value.
>
>That's perfectly fine, but it'd help avoid confusion if the supporting
>comment was a bit more direct:
>
>	/*
>	 * If no hibernate type is specified SYSTEM_OFF2 defaults to
>	 * selecting HIBERNATE_OFF.
>	 *
>	 * There are hypervisors in the wild that violate the spec and
>	 * reject calls that explicitly provide a hibernate type. For
>	 * compatibility with these nonstandard implementations, pass 0
>	 * as the type.
>	 */
>	 if (system_entering_hibernation())
>		invoke_psci_fn(PSCI_FN_NATIVE(1_3, SYSTEM_OFF2), 0 , 0, 0);

By the time this makes it into released versions of the guest Linux kernel, that comment won't be true any more.