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

[Marc Zyngier <maz@kernel.org>]

On Mon, 18 Mar 2024 18:15:36 +0000,
David Woodhouse <dwmw2@infradead.org> wrote:
> 
> [1  <text/plain; UTF-8 (quoted-printable)>]
> On Mon, 2024-03-18 at 17:41 +0000, Marc Zyngier wrote:
> > On Mon, 18 Mar 2024 17:26:07 +0000,
> > David Woodhouse <dwmw2@infradead.org> wrote:
> > > 
> > > [1  <text/plain; UTF-8 (quoted-printable)>]
> > > 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?
> > 
> > No. The *VMM* sets the PSCI version by writing to a pseudo register.
> > It means that when the guest migrates, the VMM saves and restores that
> > version, and the guest doesn't see any change.
> 
> And when you boot a guest image which has been working for years under
> a new kernel+KVM, your guest suddenly experiences a new PSCI version.
> As I said that's not just new optional functions; it's potentially even
> returning new error codes to the functions that said guest was already
> using.

If you want to stick to a given PSCI version, you write the version
you want.

> 
> And when you *hibernate* a guest and then launch it again under a newer
> kernel+KVM, it experiences the same incompatibility.
> 
> Unless the VMM realises this problem and opts *out* of the newer KVM
> behaviour, of course. This is very much unlike how we *normally* expose
> new KVM capabilities.

This was discussed at length 5 or 6 years ago (opt-in vs opt-out).

The feedback from QEMU (which is the only public VMM that does
anything remotely useful with this) was that opt-out was a better
model, specially as PSCI is the conduit for advertising the Spectre
mitigations and users (such as certain cloud vendors) were pretty keen
on guests seeing the mitigations advertised *by default*.

And if you can spot any form of "normality" in the KVM interface, I'll
buy you whatever beer you want. It is all inconsistent crap, so I
think we're in pretty good company here.

> 
> > > 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?
> > 
> > Arbitrary? No. This is actually very specific and pretty well
> > documented.
> > 
> > Also, to answer your question about why we treat 0.1 differently from
> > 0.2+: 0.1 didn't specify the PSCI SMC/HCR encoding, meaning that KVM
> > implemented something that was never fully specified. The VMM has to
> > provide firmware tables that describe that. With 0.2+, there is a
> > standard encoding for all functions, and the VMM doesn't have to
> > provide the encoding to the guest.
> 
> Gotcha. So for that case we were *forced* to do things correctly and
> allow userspace to opt-in to the capability. While for 0.2 onwards we
> got away with this awfulness of silently upgrading the version without
> VMM consent.
> 
> I was hoping to just follow the existing model of SYSTEM_RESET2 and not
> have to touch this awfulness with a barge-pole, but sure, whatever you
> want.

Unless I'm reading the whole thing wrong (which isn't impossible given
that I'm jet-lagged to my eyeballs), SYSTEM_RESET2 doesn't have any
form of configuration. If PSCI 1.1 is selected, SYSTEM_RESET2 is
available. So that'd be the model to follow.

	M.

-- 
Without deviation from the norm, progress is not possible.