Re: [Qemu-devel] [PATCH 6/8] spapr: move interrupt allocator to xics

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 04/12/2014 01:38 AM, Alexander Graf wrote:
> 
> On 11.04.14 17:27, Alexey Kardashevskiy wrote:
>> On 04/12/2014 12:58 AM, Alexander Graf wrote:
>>> On 11.04.14 16:50, Alexey Kardashevskiy wrote:
>>>> On 04/11/2014 11:58 PM, Alexander Graf wrote:
>>>>> On 11.04.2014, at 14:38, Alexey Kardashevskiy <aik@ozlabs.ru> wrote:
>>>>>
>>>>>> On 04/11/2014 07:24 PM, Alexander Graf wrote:
>>>>>>> On 10.04.14 16:43, Alexey Kardashevskiy wrote:
>>>>>>>> On 04/10/2014 11:26 PM, Alexander Graf wrote:
>>>>>>>>> On 10.04.14 15:24, Alexey Kardashevskiy wrote:
>>>>>>>>>> On 04/10/2014 10:51 PM, Alexander Graf wrote:
>>>>>>>>>>> On 14.03.14 05:18, Alexey Kardashevskiy wrote:
>>>>>>>>>>>> The current allocator returns IRQ numbers from a pool and does not
>>>>>>>>>>>> support IRQs reuse in any form as it did not keep track of what it
>>>>>>>>>>>> previously returned, it only had the last returned IRQ.
>>>>>>>>>>>> However migration may change interrupts for devices depending on
>>>>>>>>>>>> their order in the command line.
>>>>>>>>>>> Wtf? Nonono, this sounds very bogus and wrong. Migration shouldn't
>>>>>>>>>>> change
>>>>>>>>>>> anything.
>>>>>>>>>> I put wrong commit message. By change I meant that the default state
>>>>>>>>>> before
>>>>>>>>>> the destination guest started accepting migration is different from
>>>>>>>>>> what
>>>>>>>>>> the destination guest became after migration finished. And migration
>>>>>>>>>> cannot
>>>>>>>>>> avoid changing this default state.
>>>>>>>>> Ok, why is the IRQ configuration different?
>>>>>>>> Because QEMU creates devices in the order as in the command line, and
>>>>>>>> libvirt changes this order - the XML used to create the guest and the
>>>>>>>> XML
>>>>>>>> which is sends during migration are different. libvirt thinks it is ok
>>>>>>>> while it keeps @reg property for (for example) spapr-vscsi devices
>>>>>>>> but it
>>>>>>>> is not because since the order is different, devices call IRQ
>>>>>>>> allocator in
>>>>>>>> different order and get different IRQs.
>>>>>>> So your patch migrates the current IRQ configuration, but once you
>>>>>>> restart
>>>>>>> the virtual machine on the destination host it will have different IRQ
>>>>>>> numbering again, right?
>>>>>> No, why? IRQs are assigned at init time from realize() callbacks (and
>>>>>> survive reset) or as a part of ibm,change-msi rtas call which happens in
>>>>>> the same order as it only depends on pci addresses and we do not change
>>>>>> this either.
>>>>> Ok, let me rephrase. If I shut the machine down because I'm doing
>>>>> on-disk hibernate and then boot it back up, will the guest find the same
>>>>> configuration?
>>>> I do not understand what you mean by this. Hibernation by the guest OS
>>>> itself or by QEMU? If this involves QEMU exit and QEMU start - then yes,
>>> by the guest OS. The host will only see a genuine "shutdown" event. The
>>> guest OS will expect the machine to look *the exact same* as before the
>>> shutdown.
>> Ok. So. I have to implement "irq" property everywhere (PHB is missing
>> INTA/B/C/D now) and check if they did not change during migration via those
> 
> Hrm. Not sure. Maybe it'd make sense to join next week's call on platform
> device creation. The problem seems pretty closely related.

What are those platform devices and what are you going to discuss exactly?


>> VMSTATE.*EQUAL. Correct?
> 
> Why would you need this? I think we already said a couple dozen times that
> configuration matching is a bigger problem, no?

For debug! It is not needed in general, yes.


>> If so (more or less), I still would like to keep patches 1..7.
>> In fact, the first one is independent and we need it anyway.
>> Yes/no?
> 
> Why?

IOMMUs do not migrate correctly - they only have a class have and
instance_id and this instance_it depends on command line arguments order.
The #1 patch makes it classname + liobn.


> 
>>
>>
>>>> config may be different. If it is "migrate to file" and then "migrate from
>>>> file" (do not know what you call it when migration goes to a pipe which is
>>>> "tar") - then config will be the same.
>>>>
>>>>
>>>>>>> I'm not sure that's a good solution to the problem. I guess we should
>>>>>>> rather aim to make sure that we can make IRQ allocation explicit.
>>>>>>> Fundamentally the problem sounds very similar to the PCI slot
>>>>>>> allocation
>>>>>>> which eventually got solved by libvirt specifying the slots manually.
>>>>>> We can do that too. Who decides? :)
>>>>> The better solution wins :)
>>>> We both know who decides ;) I posted series, I need heads up if it is
>>>> going
>>>> the right way or not.
>>> It's not :). If a guest may not have different IRQ allocation after
>>> migration, it also must not have different IRQ allocation after shutdown +
>>> restart.
>> Ok. That's good answer, thanks. How does x86 work then? IRQs are hardcoded
>> (some are for sure but I do not know about MSI)? Or in order to support
> 
> Non-PCI IRQs are hardcoded, yes. PCI IRQs are mapped to one of the 4 PCI
> interrupts which again are hardcoded to IOAPIC interrupt lines after some
> PCI line swizzling.

This is what I meant - I need to have a way to tell PHB IRQ numbers for
INTA/B/C/D.



> MSI gets configured by the guest, so it has to make sure MSIs are set up
> identically again after hibernation.
> 
> 
> Alex
>> migration, the user has to specify IRQs for the devices which may get
>> different IRQs depending on things like command line parameters order?
> 


-- 
Alexey