Re: [RFC PATCH 0/2] Add PSCI v1.3 SYSTEM_OFF2 support for hibernation

[David Woodhouse]

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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