[Question - ARM CCA] vCPU Hotplug Support in ARM Realm world might require ARM spec change?

[Suzuki K Poulose]

Hi Salil

Thanks for raising this.

On 19/07/2023 03:35, Salil Mehta wrote:
> [Reposting it here from Linaro Open Discussion List for more eyes to look at]
> 
> Hello,
> I have recently started to dabble with ARM CCA stuff and check if our
> recent changes to support vCPU Hotplug in ARM64 can work in the realm
> world. I have realized that in the RMM specification[1] PSCI_CPU_ON
> command(B5.3.3) does not handles the PSCI_DENIED return code(B5.4.2),
> from the host. This might be required to support vCPU Hotplug feature
> in the realm world in future. vCPU Hotplug is an important feature to
> support kata-containers in realm world as it reduces the VM boot time
> and facilitates dynamic adjustment of vCPUs (which I think should be
> true even with Realm world as current implementation only makes use
> of the PSCI_ON/OFF to realize the Hotplug look-like effect?)
> 
> 
> As per our recent changes [2], [3] related to support vCPU Hotplug on
> ARM64, we handle the guest exits due to SMC/HVC Hypercall in the
> user-space i.e. VMM/Qemu. In realm world, REC Exits to host due to
> PSCI_CPU_ON should undergo similar policy checks and I think,
> 
> 1. Host should *deny* to online the target vCPUs which are NOT plugged
> 2. This means target REC should be denied by host. Can host call
>     RMI_PSCI_COMPETE in such s case?
> 3. The *return* value (B5.3.3.1.3 Output values) should be PSCI_DENIED

The Realm exit with EXIT_PSCI already provides the parameters passed
onto the PSCI request. This happens for all PSCI calls except
(PSCI_VERSION and PSCI_FEAUTRES). The hyp could forward these exits to
the VMM and could invoke the RMI_PSCI_COMPLETE only when the VMM blesses 
the request (wherever applicable).

However, the RMM spec currently doesn't allow denying the request.
i.e., without RMI_PSCI_COMPLETE, the REC cannot be scheduled back in.
We will address this in the RMM spec and get back to you.

Kind regards
Suzuki


> 4. Failure condition (B5.3.3.2) should be amended with
>     runnable pre: target_rec.flags.runnable == NOT_RUNNABLE (?)
>              post: result == PSCI_DENIED (?)
> 5. Change would also be required in the flow (D1.4 PSCI flows) depicting
>     PSCI_CPU_ON flow (D1.4.1)
>    
> 
> I do understand that ARM CCA support is in its infancy stage and
> discussing about vCPU Hotplug in realm world seem to be a far-fetched
> idea right now. But specification changes require lot of time and if
> this change is really required then it should be further discussed
> within ARM.
> 
> Many thanks!
> 
> 
> Bes regards
> Salil
> 
> 
> References:
> 
> [1] https://developer.arm.com/documentation/den0137/latest/
> [2] https://github.com/salil-mehta/qemu.git virt-cpuhp-armv8/rfc-v1-port11052023.dev-1
> [3] https://git.gitlab.arm.com/linux-arm/linux-jm.git virtual_cpu_hotplug/rfc/v2
>