Hi Jia, On 16/07/2019 08:59, Jia He wrote: > Hi Marc > > On 2019/7/10 17:15, Marc Zyngier wrote: >> On 09/07/2019 20:06, Maran Wilson wrote: >>> On 7/5/2019 3:12 AM, James Morse wrote: >>>> Hi guys, >>>> >>>> (CC: +kvmarm list) >>>> >>>> On 29/06/2019 03:42, Xiongfeng Wang wrote: >>>>> This patchset mark all the GICC node in MADT as possible CPUs even though it >>>>> is disabled. But only those enabled GICC node are marked as present CPUs. >>>>> So that kernel will initialize some CPU related data structure in advance before >>>>> the CPU is actually hot added into the system. This patchset also implement >>>>> 'acpi_(un)map_cpu()' and 'arch_(un)register_cpu()' for ARM64. These functions are >>>>> needed to enable CPU hotplug. >>>>> >>>>> To support CPU hotplug, we need to add all the possible GICC node in MADT >>>>> including those CPUs that are not present but may be hot added later. Those >>>>> CPUs are marked as disabled in GICC nodes. >>>> ... what do you need this for? >>>> >>>> (The term cpu-hotplug in the arm world almost never means hot-adding a new package/die to >>>> the platform, we usually mean taking CPUs online/offline for power management. e.g. >>>> cpuhp_offline_cpu_device()) >>>> >>>> It looks like you're adding support for hot-adding a new package/die to the platform ... >>>> but only for virtualisation. >>>> >>>> I don't see why this is needed for virtualisation. The in-kernel irqchip needs to know >>>> these vcpu exist before you can enter the guest for the first time. You can't create them >>>> late. At best you're saving the host scheduling a vcpu that is offline. Is this really a >>>> problem? >>>> >>>> If we moved PSCI support to user-space, you could avoid creating host vcpu threads until >>>> the guest brings the vcpu online, which would solve that problem, and save the host >>>> resources for the thread too. (and its acpi/dt agnostic) >>>> >>>> I don't see the difference here between booting the guest with 'maxcpus=1', and bringing >>>> the vcpu online later. The only real difference seems to be moving the can-be-online >>>> policy into the hypervisor/VMM... >>> Isn't that an important distinction from a cloud service provider's >>> perspective? >>> >>> As far as I understand it, you also need CPU hotplug capabilities to >>> support things like Kata runtime under Kubernetes. i.e. when >>> implementing your containers in the form of light weight VMs for the >>> additional security ... and the orchestration layer cannot determine >>> ahead of time how much CPU/memory resources are going to be needed to >>> run the pod(s). >> Why would it be any different? You can pre-allocate your vcpus, leave >> them parked until some external agent decides to signal the container >> that it it can use another bunch of CPUs. At that point, the container >> must actively boot these vcpus (they aren't going to come up by magic). >> >> Given that you must have sized your virtual platform to deal with the >> maximum set of resources you anticipate (think of the GIC >> redistributors, for example), I really wonder what you gain here. > I agree with your point in GIC aspect. It will mess up things if it makes > > GIC resource hotpluggable in qemu. It is far worse than just a mess. You'd need to come up with a way to place your redistributors in memory, and tell the running guest where these redistributors are. Currently, there is no method to describe such changes to the address space, and I certainly don't want QEMU to invent one. This needs to be modeled after what would happen on real HW. > But it also would be better that vmm > > only startup limited vcpu thread resource. > > How about: > > 1. qemu only starts only N vcpu thread (-smp N, maxcpus=M) > > 2. qemu reserves the GIC resource with maxium M vcpu number Note that this implies actually initializing M vcpus in the VM. You may not have created the corresponding (M - N) threads, but the vcpus will exist. Can you please quantify how much you'd save by doing that? > 3. when qmp cmd cpu hotplug-add is triggerred, send a GED event to guest kernel > > 4. guest kernel recv it and trigger the acpi plug process. > > Currently ACPI_CPU_HOTPLUG is enabled for Kconfig but completely not workable. Well, there so far *zero* CPU_HOTPLUG in the arm64 kernel other than getting CPUs in and out of PSCI. Thanks, M. -- Jazz is not dead. It just smells funny...
