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

[Suzuki K Poulose]

Hi Salil

On 25/07/2023 01:05, Salil Mehta wrote:
> Hi Suzuki,
> Sorry for replying late as I was on/off last week to undergo some medical test.
> 
> 
>> From: Suzuki K Poulose <suzuki.poulose at arm.com>
>> Sent: Monday, July 24, 2023 5:27 PM
>>
>> 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/
> 
> 
> Many thanks for taking this up proactively and getting it done as well
> very efficiently. Really appreciate this!
> 
> I acknowledge below new changes part of the newly released RMM
> Specification [3] (Page-2) (Release Information 1.0-eac3 20-07-2023):
> 
> 1. Addition of B2.19 PsciReturnCodePermitted function [3] (Page-126)
> 2. Addition of 'status' in B3.3.7.2 Failure conditions of the
>     B3.3.7 RMI_PSCI_COMPLETE command [3] (Page-160)
> 
> 
> Some Further Suggestions:
> 1. It would be really helpful if PSCI_DENIED can be accommodated somewhere
>     in the flow diagram (D1.4.1 PSCI_CPU_ON flow) [3] (Page-297) as well.

Good point, yes, will get that added.

> 2. You would need changes to handle the return value of the PSCI_DENIED
>     in this below patch [2] as well from ARM CCA series [1]
> 

Of course. Please note that the series [1] is based on RMMv1.0-beta0 and
we are in the process of rebasing our changes to v1.0-eac3, which
includes a lot of other changes. The updated series will have all the
required changes.

Kind regards
Suzuki


> 
> @James, Any further thoughts on this?
> 
> 
> References:
> [1] [RFC PATCH 00/28] arm64: Support for Arm CCA in KVM
> [2] [RFC PATCH 19/28] KVM: arm64: Validate register access for a Realm VM
>       https://lore.kernel.org/lkml/20230127112248.136810-1-suzuki.poulose@arm.com/T/#m6c10b9a27c4a724967c1800facacaa9443b38b4c
> [3] ARM Realm Management Monitor specification(DEN0137 1.0-eac3)
>      https://developer.arm.com/documentation/den0137/latest/
>   
> 
> Thanks
> Salil.
> 
> 
>>>> 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
>