RE: [PATCH] cpu/hotplug: Allow the CPU in CPU_UP_PREPARE state to be brought up again.

[Valentin Schneider <valentin.schneider@arm.com>]

On 24/11/21 00:19, Longpeng (Mike, Cloud Infrastructure Service Product Dept.) wrote:
>> -----Original Message-----
>> From: Valentin Schneider [mailto:valentin.schneider@arm.com]
>> For my own education, this is talking about *host* CPU hotplug, right?
>>
>
> It's about the *guest* CPU hotplug.
>
> 1. Users in Primary VM:
> Split out vcpuX (offline from Primary VM) for Enclave VM
>
> 2. Hypervisor:
> Set a flag for vcpuX, all requests from Primary VM to bring up vcpuX
> will be ignore.
>
> 3. Users in Primary VM:
> echo 1 > .../vcpuX/online would fail and leave the CPU state of vcpuX
> in CPU_UP_PREPARE.
>
> 4. Users in Primary VM terminate the Enclave VM:
> Hypervisor should clear the flag (set in step 2) of vcpuX, so the vcpuX
> can continue to receive requests.
>
> 5. Users in Primary VM:
> Try to online the vcpuX again (expect success), but it's always failed.
>

If I followed the rabbit hole in the right direction, this is about:
ff8a4d3e3a99 ("nitro_enclaves: Add logic for setting an enclave vCPU")

So there's a 1:1 CPU:vCPU mapping and an Enclave carves a chunk out of the
Primary VM...

Thanks for the detailed explanation!