Re: [RFC PATCH v2 2/4] KVM: arm64: Add PSCI SYSTEM_OFF2 function for hibernation

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On Mon, 2024-03-18 at 18:07 +0000, Marc Zyngier wrote:
> On Mon, 18 Mar 2024 17:54:06 +0000,
> David Woodhouse <dwmw2@xxxxxxxxxxxxx> wrote:
> > 
> > [1  <text/plain; UTF-8 (quoted-printable)>]
> > On Mon, 2024-03-18 at 17:29 +0000, Marc Zyngier wrote:
> > > 
> > > Again, I really oppose this way of doing things. We already have an
> > > infrastructure for selecting PSCI levels. You may not like it, but it
> > > exists, and I'm not going entertain supporting yet another bike-shed
> > > model. Adding an orthogonal cap for a feature that is specific to a
> > > new PSCI version is just awful.
> > 
> > Huh? This isn't a "new bike-shed model". This is a straight copy of
> > what we *already* have for SYSTEM_RESET2.
> There is no KVM capability for SYSTEM_RESET2. It is directly
> advertised to the guest when PSCI 1.1 is supported.

Apologies, I got that wrong. It's SYSTEM_SUSPEND and the corresponding
mixed those up.
> > If I were bike-shedding, I wouldn't do separate caps for them; I'd have
> > done it as a *bitmask* of the optional PSCI calls that should be
> > enabled.
> > 
> > The *mandatory* ones should obviously come from the PSCI version alone,
> > but I can't see how that makes sense for the optional ones...
> The guest is in a position to probe for what is supported or not with
> the PSCI_FEATURES call.  Why would you add anything else?

Because we don't want to silently *change* what's advertised to the
guest with the VMM explicitly opting in. 

> > > Please make PSCI 1.3 the only version of PSCI supporting suspend in a
> > > non-optional way, and be done with it.
> > 
> > SYSTEM_OFF2 is an *optional* feature in PSCI v1.3. As are
> > 
> > Are you suggesting that enabling v1.3 should automatically enable *all*
> > of the optional features that were defined in that version (and
> > previous versions) of the spec?
> No. We have everything we need to incrementally *add* features. So you
> can perfectly implement PSCI 1.3 with only SYSTEM_OFF2, and only later
> on add the rest, if ever.

OK. It's still awful, but I suppose can live with that since existing
VMMs will just see the same KVM_SYSTEM_EVENT_SHUTDOWN as before, and
hopefully just won't understand the flag (and won't notice) the extra
flag which says it's a hibernate.

A VMM might *perhaps* check for flags it doesn't understand and
complain about them, which is why we shouldn't really do that. But
where PSCI is concerned it seems we've left best practice behind a long
time ago, so I'll let it go.

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