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

[bibo mao]

在 2023/7/25 08:56, Salil Mehta 写道:
> Hi Bibo,
> 
>> From: bibo mao <maobibo at loongson.cn>
>> Sent: Tuesday, July 25, 2023 1:29 AM
>> To: Salil Mehta <salil.mehta at huawei.com>
>> Cc: Catalin Marinas <catalin.marinas at arm.com>; Jonathan Cameron
>> <jonathan.cameron at huawei.com>; Marc Zyngier <maz at kernel.org>; Will Deacon
>> <will at kernel.org>; christoffer.dall at arm.com; oliver.upton at linux.dev;
>> mark.rutland at arm.com; pbonzini at redhat.com; Salil Mehta
>> <salil.mehta at opnsrc.net>; andrew.jones at linux.dev; yuzenghui
>> <yuzenghui at huawei.com>; kvmarm at lists.cs.columbia.edu; linux-arm-
>> kernel at lists.infradead.org; linux-kernel at vger.kernel.org;
>> kvm at vger.kernel.org; qemu-devel at nongnu.org; james.morse at arm.com;
>> steven.price at arm.com; Suzuki K Poulose <suzuki.poulose at arm.com>; Jean-
>> Philippe Brucker <jean-philippe at linaro.org>; kvmarm at lists.linux.dev; linux-
>> coco at lists.linux.dev
>> Subject: Re: [Question - ARM CCA] vCPU Hotplug Support in ARM Realm world
>> might require ARM spec change?
>>
>> Is vcpu hotplug supported in arm virt-machine now?
> 
> Not yet. We are working on it. Please check the RFCs being tested.
> 
> 
> [1] Pre-RFC V2 Changes: Support of Virtual CPU Hotplug for ARMv8 Arch  (WIP)
>     https://github.com/salil-mehta/qemu.git virt-cpuhp-armv8/rfc-v1-port11052023.dev-1
> [2] [RFC PATCH 00/32] ACPI/arm64: add support for virtual cpuhotplug  
>     https://git.gitlab.arm.com/linux-arm/linux-jm.git virtual_cpu_hotplug/rfc/v2
>     
>     
>> There is arm64 vcpu hotplug patch in qemu mailing list, however it is not merged.
>> I do not know why it is not merged.
> 
> 
> I think you are referring to patches [3], [4]? Please follow the discussion
> for details. 
yeap, we reference the patch [3], [4] and benefit from them greatly -:)

The patch for LoongArch vcpu hotplug link is:
https://lore.kernel.org/qemu-devel/cover.1689837093.git.lixianglai@loongson.cn/T/#t


Regards
Bibo Mao

> 
> 
> [3] [PATCH RFC 00/22] Support of Virtual CPU Hotplug for ARMv8 Arch
>     https://lore.kernel.org/all/20200613213629.21984-1-salil.mehta@huawei.com/
> [4] [PATCH RFC 0/4] Changes to Support *Virtual* CPU Hotplug for ARM64
>     https://lore.kernel.org/all/20200625133757.22332-1-salil.mehta@huawei.com/#r
> 
> 
> In summary, there were some ARM64 Architecture constraints which were being
> violated in the earlier patches of the kernel [4] so we had to re-think of the
> kernel changes. The Qemu part mostly remains same with some new introductions
> of Guest HVC/SMC hyper call exit handling in user space etc. for policy checks
> in VMM/Qemu. 
> 
> 
> You can follow the KVMForum conference presentations [5], [6] delivered in the
> year 2020 and 2023 to get hold of more details related to this.
> 
> 
> [5] KVMForum 2023: Challenges Revisited in Supporting Virt CPU Hotplug on architectures that don't Support CPU Hotplug (like ARM64)
>     https://kvm-forum.qemu.org/2023/talk/9SMPDQ/
> [6] KVMForum 2020: Challenges in Supporting Virtual CPU Hotplug on SoC Based Systems (like ARM64)
>     https://kvmforum2020.sched.com/event/eE4m
> 
> 
> 
>> I ask this question because we propose
>> similar patch about LoongArch system in qemu mailing list, and kernel need not be
>> modified for vcpu hotplug.
> 
> 
> Could you please share the link of your patches so that we can have a look and
> draw a comparison?
> 
> 
> Thanks
> Salil.
> 
>>
>> Regards
>> Bibo, mao
>>
>> 在 2023/7/19 10:35, Salil Mehta 写道:
>>> [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
>>> 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