linux-kernel - Re: [RFC PATCH 0/2] Add PSCI v1.3 SYSTEM

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Date: Tue, 12 Mar 2024 17:01:10 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: Marc Zyngier <maz@...nel.org>
Cc: linux-arm-kernel@...ts.infradead.org, kvm@...r.kernel.org, Paolo Bonzini
 <pbonzini@...hat.com>, Jonathan Corbet <corbet@....net>, Oliver Upton
 <oliver.upton@...ux.dev>, 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>, Len Brown
 <len.brown@...el.com>, Pavel Machek <pavel@....cz>, Mostafa Saleh
 <smostafa@...gle.com>, Jean-Philippe Brucker <jean-philippe@...aro.org>, 
 linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
 kvmarm@...ts.linux.dev,  linux-pm@...r.kernel.org
Subject: Re: [RFC PATCH 0/2] Add PSCI v1.3 SYSTEM_OFF2 support for
 hibernation

On Tue, 2024-03-12 at 15:24 +0000, Marc Zyngier wrote:
> 
> > Strictly, we should perhaps also allow the guest to detect PSCI v1.3, 
> > but when v1.1 was added in commit 512865d83fd9 it was done 
> > unconditionally, which seems wrong. Shouldn't we have a way for 
> > userspace to control what gets exposed, rather than silently changing 
> > the guest behaviour with newer host kernels? Should I add a 
> > KVM_CAP_ARM_PSCI_VERSION?
> 
> Do you mean something like 85bd0ba1ff98?

Ew :)

That isn't quite what I was thinking, no. I wasn't thinking of
something that would default to the latest, and would have a per-vCPU
way of setting what's essentially a KVM-wide configuration.

So if current userspace doesn't want the environment it exposes to
guests to be randomly changed by a kernel upgrade in the future, it
needs to explicitly use KVM_ARM_SET_REG on any one of the vCPUs, to set
KVM_REG_ARM_PSCI_VERSION to KVM_ARM_PSCI_1_1?

It isn't just new optional features; PSCI v1.2 added new error returns
from CPU_ON for example. Should guests start to see those, just because
the host kernel got upgraded? 

Now I see it, I suppose we can extend it to v1.2 (and v1.3 when that's
eventually published for real). Should we really continue to increment
the *default* though?

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