Re: [PATCH RFC] vfio: Move the saving of the config space to the right place in VFIO migration

[Shenming Lu]

On 2020/11/21 6:01, Alex Williamson wrote:
> On Fri, 20 Nov 2020 22:05:49 +0800
> Shenming Lu <lushenming@huawei.com> wrote:
> 
>> On 2020/11/20 1:41, Alex Williamson wrote:
>>> On Thu, 19 Nov 2020 14:13:24 +0530
>>> Kirti Wankhede <kwankhede@nvidia.com> wrote:
>>>   
>>>> On 11/14/2020 2:47 PM, Shenming Lu wrote:  
>>>>> When running VFIO migration, I found that the restoring of VFIO PCI 
>>>>> device’s
>>>>> config space is before VGIC on ARM64 target. But generally, interrupt 
>>>>> controllers
>>>>> need to be restored before PCI devices.     
>>>>
>>>> Is there any other way by which VGIC can be restored before PCI device?  
>>
>> As far as I know, it seems to have to depend on priorities in the 
>> non-iterable process.
>>
>>>>  
>>>>> Besides, if a VFIO PCI device is
>>>>> configured to have directly-injected MSIs (VLPIs), the restoring of its 
>>>>> config
>>>>> space will trigger the configuring of these VLPIs (in kernel), where it 
>>>>> would
>>>>> return an error as I saw due to the dependency on kvm’s vgic.
>>>>>     
>>>>
>>>> Can this be fixed in kernel to re-initialize the kernel state?  
>>
>> Did you mean to reconfigure these VLPIs when restoring kvm's vgic?
>> But the fact is that this error is not caused by kernel, it is due to the 
>> incorrect
>> calling order of qemu...
>>
>>>>  
>>>>> To avoid this, we can move the saving of the config space from the 
>>>>> iterable
>>>>> process to the non-iterable process, so that it will be called after VGIC
>>>>> according to their priorities.
>>>>>     
>>>>
>>>> With this change, at resume side, pre-copy phase data would reach 
>>>> destination without restored config space. VFIO device on destination 
>>>> might need it's config space setup and validated before it can accept 
>>>> further VFIO device specific migration state.
>>>>
>>>> This also changes bit-stream, so it would break migration with original 
>>>> migration patch-set.  
>>>
>>> Config space can continue to change while in pre-copy, if we're only
>>> sending config space at the initiation of pre-copy, how are any changes
>>> that might occur before the VM is stopped conveyed to the target?  For
>>> example the guest might reboot and a device returned to INTx mode from
>>> MSI during pre-copy.  Thanks,  
>>
>> What I see is that the config space is only saved once in 
>> save_live_complete_precopy
>> currently...
>> As you said, a VFIO device might need it's config space setup first, and
>> the config space can continue to change while in pre-copy, Did you mean we
>> have to migrate the config space in save_live_iterate?
>> However, I still have a little doubt about the restoring dependence between
>> the qemu emulated config space and the device data...
>>
>> Besides, if we surely can't move the saving of the config space back, can we
>> just move some actions which are triggered by the restoring of the config 
>> space
>> back (such as vfio_msix_enable())?
> 
> It seems that the significant benefit to enabling interrupts during
> pre-copy would be to reduce the latency and failure potential during
> the final phase of migration.  Do we have any data for how much it adds
> to the device contributed downtime to configure interrupts only at the
> final stage?  My guess is that it's a measurable delay on its own.  At
> the same time, we can't ignore the differences in machine specific
> dependencies and if we don't even sync the config space once the VM is
> stopped... this all seems not ready to call supported, especially if we
> have concerns already about migration bit-stream compatibility.
> 

I have another question for this, if we restore the config space while in 
pre-copy
(include enabling interrupts), does it affect the _RESUMING state (paused) of 
the
device on the dst host (cause it to send interrupts? which should not be allowed
in this stage). Does the restore sequence need to be further discussed and reach
a consensus(spec) (taking into account other devices and the corresponding 
actions
of the vendor driver)?

> Given our timing relative to QEMU 5.2, the only path I feel comfortable
> with is to move forward with downgrading vfio migration support to be
> enabled via an experimental option.  Objections?  Thanks,

Alright, but this issue is related to our ARM GICv4.1 migration scheme, could 
you
give a rough idea about this (where to enable interrupts, we hope it to be after
the restoring of VGIC)?

Thanks,
Shenming