Re: [PATCH 4/9] cpuidle-haltpoll: define arch_haltpoll_supported()

From: Joao Martins <joao.m.martins@oracle.com>
To: Ankur Arora <ankur.a.arora@oracle.com>
Cc: catalin.marinas@arm.com, will@kernel.org, tglx@linutronix.de,
	mingo@redhat.com, bp@alien8.de, x86@kernel.org, hpa@zytor.com,
	pbonzini@redhat.com, wanpengli@tencent.com, vkuznets@redhat.com,
	rafael@kernel.org, daniel.lezcano@linaro.org,
	peterz@infradead.org, arnd@arndb.de, lenb@kernel.org,
	mark.rutland@arm.com, harisokn@amazon.com,
	boris.ostrovsky@oracle.com, konrad.wilk@oracle.com,
	linux-kernel@vger.kernel.org, linux-pm@vger.kernel.org,
	kvm@vger.kernel.org, linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 4/9] cpuidle-haltpoll: define arch_haltpoll_supported()
Date: Wed, 22 May 2024 17:09:20 +0100	[thread overview]
Message-ID: <3f72519e-bf82-458f-a066-0c296a7d655f@oracle.com> (raw)
In-Reply-To: <20240430183730.561960-5-ankur.a.arora@oracle.com>

On 30/04/2024 19:37, Ankur Arora wrote:
> From: Joao Martins <joao.m.martins@oracle.com>
> 
> Right now kvm_para_has_hint(KVM_HINTS_REALTIME) is x86 only. In
> pursuit of making cpuidle-haltpoll architecture independent, define
> arch_haltpoll_supported() which handles the architectural check for
> enabling haltpoll.
> 
> Move the (kvm_para_available() && kvm_para_has_hint(KVM_HINTS_REALTIME))
> check to the x86 specific arch_haltpoll_supported().
> 
> Signed-off-by: Joao Martins <joao.m.martins@oracle.com>
> Signed-off-by: Mihai Carabas <mihai.carabas@oracle.com>
> Signed-off-by: Ankur Arora <ankur.a.arora@oracle.com>
> 
> ---
> Changelog:
> 
>   - s/arch_haltpoll_want/arch_haltpoll_supported/

I am not sure it's correct to call supported() considering that it's supposed to
always supported (via WFE or cpu_relax()) and it's not exactly what it is doing.
The function you were changing is more about whether it's default enabled or
not. So I think the old name in v4 is more appropriate i.e. arch_haltpoll_want()

Alternatively you could have it called arch_haltpoll_default_enabled() though
it's longer/verbose.

Though if you want a true supported() arch helper *I think* you need to make a
bigger change into introducing arch_haltpoll_supported() separate from
arch_haltpoll_want() where the former would ignore the .force=y modparam and
never be able to load if a given feature wasn't present e.g. prevent arm64
haltpoll loading be conditioned to arch_timer_evtstrm_available() being present.
Though I don't think that you want this AIUI

	Joao

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