On 07/14/20 17:19, Igor Mammedov wrote: > On Tue, 14 Jul 2020 14:41:28 +0200 > Laszlo Ersek <ler...@redhat.com> wrote: > >> On 07/14/20 14:28, Laszlo Ersek wrote: >>> (CC'ing Peter Krempa due to virsh setvcpu (singular) / setvcpus (plural) >>> references) >>> >>> On 07/10/20 18:17, Igor Mammedov wrote: >>>> In case firmware has negotiated CPU hotplug SMI feature, generate >>>> AML to describe SMI IO port region and send SMI to firmware >>>> on each CPU hotplug SCI. >>>> >>>> It might be not really usable, but should serve as a starting point to >>>> discuss how better to deal with split hotplug sequence during hot-add >>>> ( >>>> ex scenario where it will break is: >>>> hot-add >>>> -> (QEMU) add CPU in hotplug regs >>>> -> (QEMU) SCI >>>> -1-> (OS) scan >>>> -1-> (OS) SMI >>>> -1-> (FW) pull in CPU1 *** >>>> -1-> (OS) start iterating hotplug regs >>>> hot-add >>>> -> (QEMU) add CPU in hotplug regs >>>> -> (QEMU) SCI >>>> -2-> (OS) scan (blocked on mutex till previous scan is >>>> finished) >>>> -1-> (OS) 1st added CPU1 send device check event -> >>>> INIT/SIPI >>>> -1-> (OS) 1st added CPU2 send device check event -> >>>> INIT/SIPI >>>> that's where it explodes, since FW didn't see CPU2 >>>> when SMI was called >>>> ) >>>> >>>> hot remove will throw in yet another set of problems, so lets discuss >>>> both here and see if we can really share hotplug registers block between >>>> FW and AML or we should do something else with it. >>> >>> This issue is generally triggered by management applications such as >>> libvirt that issue device_add commands in quick succession. For libvirt, >>> the command is "virsh setvcpus" (plural) with multiple CPUs specified >>> for plugging. The singular "virsh setvcpu" command, which is more >>> friendly towards guest NUMA, does not run into the symptom. >>> >>> The scope of the scan method lock is not large enough, with SMI in the >>> picture. >>> >>> I suggest that we not uproot the existing AML code or the hotplug >>> register block. Instead, I suggest that we add serialization at a higher >>> level, with sufficient scope. >>> >>> QEMU: >>> >>> - introduce a new flag standing for "CPU plug operation in progress" >>> >>> - if ICH9_LPC_SMI_F_BROADCAST_BIT has been negotiated: >>> >>> - "device_add" and "device_del" should enforce >>> ICH9_LPC_SMI_F_CPU_HOTPLUG_BIT and >>> ICH9_LPC_SMI_F_CPU_HOT_UNPLUG_BIT, respectively >>> >>> - both device_add and device_del (for VCPUs) should set check the >>> "in progress" flag. >>> >>> - If set, reject the request synchronously >>> >>> - Otherwise, set the flag, and commence the operation >>> >>> - in cpu_hotplug_wr(), where we emit the ACPI_DEVICE_OST event with >>> qapi_event_send_acpi_device_ost(), clear the "in-progress" flag. >>> >>> - If QEMU executes the QMP command processing and the cpu_hotplug_wr() >>> function on different (host OS) threads, then perhaps we should use an >>> atomic type for the flag. (Not sure about locking between QEMU threads, >>> sorry.) I don't really expect race conditions, but in case we ever get >>> stuck with the flag, we should make sure that the stuck state is "in >>> progress", and not "not in progress". (The former state can prevent >>> further plug operations, but cannot cause the guest to lose state.) >> >> Furthermore, the "CPU plug operation in progress" flag should be: >> - either migrated, >> - or a migration blocker. >> >> Because on the destination host, device_add should be possible if and >> only if the plug operation completed (either still on the source host, >> or on the destination host). > > I have a way more simple alternative idea, which doesn't involve libvirt. > > We can change AML to > 1. cache hotplugged CPUs from controller > 2. send SMI > 3. send Notify event to OS to online cached CPUs > this way we never INIT/SIPI cpus that FW hasn't seen yet > as for FW, it can relocate extra CPU that arrived after #1 > it won't cause any harm as on the next SCI AML will pick up those > CPUs and SMI upcall will be just NOP. > > I'll post a patch here on top of this series for you to try > (without any of your comments addressed yet, as it's already written > and I was testing it for a while to make sure it won't explode > with various windows versions)
Sounds good, I'll be happy to test it. Indeed "no event" is something that the fw deals with gracefully. (IIRC I wanted to cover a "spurious SMI" explicitly.) It didn't occur to me that you could dynamically size e.g. a package object in AML. Based on my reading of the ACPI spec, "VarPackageOp" can take a *runtime* "NumElements", so if you did two loops, the first loop could count the pending stuff, and then a VarPackageOp could be used with just the right NumElements... Anyway, I digress :) > >> I guess that the "migration blocker" option is easier. >> >> Anyway I assume this is already handled with memory hotplug somehow >> (i.e., migration attempt between device_add and ACPI_DEVICE_OST). > > Thanks for comments, > I'll need some time to ponder on other comments and do some > palaeontology research to answer questions > (aka. I need to make up excuses for the code I wrote :) ) haha, thanks :) Laszlo
