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

[Suzuki K Poulose]

Hi Salil

On 19/07/2023 10:28, Suzuki K Poulose wrote:
> 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.

This is now resolved in RMMv1.0-eac3 spec, available here [0].

This allows the host to DENY a PSCI_CPU_ON request. The RMM ensures that
the response doesn't violate the security guarantees by checking the
state of the target REC.

[0] https://developer.arm.com/documentation/den0137/latest/

Kind regards
Suzuki




> 
> 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
>>
>