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Re: [RFC 3/3] x68: acpi: trigger SMI before scanning for hotplugged CPUs
From: |
Laszlo Ersek |
Subject: |
Re: [RFC 3/3] x68: acpi: trigger SMI before scanning for hotplugged CPUs |
Date: |
Thu, 16 Jul 2020 14:36:51 +0200 |
User-agent: |
Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Firefox/52.0 Thunderbird/52.9.1 |
On 07/15/20 15:43, Igor Mammedov wrote:
> On Wed, 15 Jul 2020 14:38:00 +0200
> Laszlo Ersek <lersek@redhat.com> wrote:
>
>> On 07/14/20 17:19, Igor Mammedov wrote:
>>> On Tue, 14 Jul 2020 14:41:28 +0200
>>> Laszlo Ersek <lersek@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.)
> is it possible to distinguish "spurious SMI" vs hotplug SMI,
> if yes then we probably do not care about any other SMIs except hotplug one.
Sorry, I was unclear. By "spurious SMI", I meant that the hotplug SMI
(command value 4) is raised, but then the hotplug register block reports
no CPUs with pending events. The fw code handles that.
>
>> 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 :)
>
> well, it's mine field since Windows implement only a subset of spec
> and VarPackageOp is not avalable on all version that support hotplug.
Ugh. :/
> I think, I've narrowed language down to supported subset, so I need
> to complete another round of testing to see if I didn't break anything
> by accident.
Thanks.
Laszlo
>
>>>
>>>> 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
>
[RFC 1/3] x86: lpc9: let firmware negotiate CPU hotplug SMI feature, Igor Mammedov, 2020/07/10
Re: [RFC 0/3] x86: fix cpu hotplug with secure boot, Laszlo Ersek, 2020/07/14
Re: [RFC 0/3] x86: fix cpu hotplug with secure boot, Laszlo Ersek, 2020/07/14