Re: [RFC PATCH v2 0/4] arm64: Add PSCI v1.3 SYSTEM_OFF2 support for hibernation

From: David Woodhouse <dwmw2@infradead.org>
To: Marc Zyngier <maz@kernel.org>
Cc: linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org,
	Paolo Bonzini <pbonzini@redhat.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Oliver Upton <oliver.upton@linux.dev>,
	James Morse <james.morse@arm.com>,
	Suzuki K Poulose <suzuki.poulose@arm.com>,
	Zenghui Yu <yuzenghui@huawei.com>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>,
	Mark Rutland <mark.rutland@arm.com>,
	Lorenzo Pieralisi <lpieralisi@kernel.org>,
	"Rafael J. Wysocki" <rafael@kernel.org>,
	Len Brown <len.brown@intel.com>, Pavel Machek <pavel@ucw.cz>,
	Mostafa Saleh <smostafa@google.com>,
	Jean-Philippe Brucker <jean-philippe@linaro.org>,
	 linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	kvmarm@lists.linux.dev,  linux-pm@vger.kernel.org
Subject: Re: [RFC PATCH v2 0/4] arm64: Add PSCI v1.3 SYSTEM_OFF2 support for hibernation
Date: Mon, 18 Mar 2024 17:26:07 +0000	[thread overview]
Message-ID: <eb9215850e8231ab8ef75f523925be671cc6f5a0.camel@infradead.org> (raw)
In-Reply-To: <86wmpzzdep.wl-maz@kernel.org>

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On Mon, 2024-03-18 at 16:57 +0000, Marc Zyngier wrote:
> 
> > 
> > There *is* a way for a VMM to opt *out* of newer PSCI versions... by 
> > setting a per-vCPU "special" register that actually ends up setting the 
> > PSCI version KVM-wide. Quite why this isn't just a simple KVM_CAP, I 
> > have no idea.
> 
> Because the expectations are that the VMM can blindly save/restore the
> guest's state, including the PSCI version, and restore that blindly.
> KVM CAPs are just a really bad design pattern for this sort of things.

Hm, am I missing something here? Does the *guest* get to set the PSCI
version somehow, and opt into the latest version that it understands
regardless of what the firmware/host can support?

Because if not, surely it's just part of the basic shape of the
machine, like "how many vCPUs does it have". You don't need to be able
to query it back again. 

I don't think we ever aspired to be able to hand an arbitrary KVM fd to
a userspace VMM and have the VMM be able to drive that VM without
having any a priori context, did we?

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