On Mon, 2024-03-18 at 18:31 +0000, Marc Zyngier wrote: > On Mon, 18 Mar 2024 18:15:36 +0000, > David Woodhouse <dwmw2@xxxxxxxxxxxxx> 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@xxxxxxxxxxxxx> 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*. OK. > 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'll give you that one :) > > > > > > 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. Sorry, that was supposed to be SYSTEM_SUSPEND not SYSTEM_RESET2. But OK.
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