Re: [PATCH v10 Kernel 1/5] vfio: KABI for migration interface for device state

[Kirti Wankhede <kwankhede@nvidia.com>]

On 1/7/2020 10:39 PM, Alex Williamson wrote:
> On Tue, 7 Jan 2020 12:58:22 +0530
> Kirti Wankhede <kwankhede@nvidia.com> wrote:
> 
>> On 1/7/2020 4:48 AM, Alex Williamson wrote:
>>> On Thu, 2 Jan 2020 18:25:37 +0000
>>> "Dr. David Alan Gilbert" <dgilbert@redhat.com> wrote:
>>>    
>>>> * Alex Williamson (alex.williamson@redhat.com) wrote:
>>>>> On Fri, 20 Dec 2019 01:40:35 +0530
>>>>> Kirti Wankhede <kwankhede@nvidia.com> wrote:
>>>>>       
>>>>>> On 12/19/2019 10:57 PM, Alex Williamson wrote:
>>>>>>
>>>>>> <Snip>
>>>>>>       
>>>>
>>>> <snip>
>>>>   
>>>>>>
>>>>>> If device state it at pre-copy state (011b).
>>>>>> Transition, i.e., write to device state as stop-and-copy state (010b)
>>>>>> failed, then by previous state I meant device should return pre-copy
>>>>>> state(011b), i.e. previous state which was successfully set, or as you
>>>>>> said current state which was successfully set.
>>>>>
>>>>> Yes, the point I'm trying to make is that this version of the spec
>>>>> tries to tell the user what they should do upon error according to our
>>>>> current interpretation of the QEMU migration protocol.  We're not
>>>>> defining the QEMU migration protocol, we're defining something that can
>>>>> be used in a way to support that protocol.  So I think we should be
>>>>> concerned with defining our spec, for example my proposal would be: "If
>>>>> a state transition fails the user can read device_state to determine the
>>>>> current state of the device.  This should be the previous state of the
>>>>> device unless the vendor driver has encountered an internal error, in
>>>>> which case the device may report the invalid device_state 110b.  The
>>>>> user must use the device reset ioctl in order to recover the device
>>>>> from this state.  If the device is indicated in a valid device state
>>>>> via reading device_state, the user may attempt to transition the device
>>>>> to any valid state reachable from the current state."
>>>>
>>>> We might want to be able to distinguish between:
>>>>     a) The device has failed and needs a reset
>>>>     b) The migration has failed
>>>
>>> I think the above provides this.  For Kirti's example above of
>>> transitioning from pre-copy to stop-and-copy, the device could refuse
>>> to transition to stop-and-copy, generating an error on the write() of
>>> device_state.  The user re-reading device_state would allow them to
>>> determine the current device state, still in pre-copy or failed.  Only
>>> the latter would require a device reset.
>>>    
>>>> If some part of the devices mechanics for migration fail, but the device
>>>> is otherwise operational then we should be able to decide to fail the
>>>> migration without taking the device down, which might be very bad for
>>>> the VM.
>>>> Losing a VM during migration due to a problem with migration really
>>>> annoys users; it's one thing the migration failing, but taking the VM
>>>> out as well really gets to them.
>>>>
>>>> Having the device automatically transition back to the 'running' state
>>>> seems a bad idea to me; much better to tell the hypervisor and provide
>>>> it with a way to clean up; for example, imagine a system with multiple
>>>> devices that are being migrated, most of them have happily transitioned
>>>> to stop-and-copy, but then the last device decides to fail - so now
>>>> someone is going to have to take all of them back to running.
>>>
>>> Right, unless I'm missing one, it seems invalid->running is the only
>>> self transition the device should make, though still by way of user
>>> interaction via the reset ioctl.  Thanks,
>>>    
>>
>> Instead of using invalid state by vendor driver on device failure, I
>> think better to reserve one bit in device state which vendor driver can
>> set on device failure. When error bit is set, other bits in device state
>> should be ignored.
> 
> Why is a separate bit better?  Saving and Restoring states are mutually
> exclusive, so we have an unused and invalid device state already
> without burning another bit.  Thanks,
> 

There are 3 invalid states:
  *  101b => Invalid state
  *  110b => Invalid state
  *  111b => Invalid state

why only 110b should be used to report error from vendor driver to 
report error? Aren't we adding more confusions in the interface?

Only 3 bits from 32 bits are yet used, one bit can be spared to 
represent error state from vendor driver.

Thanks,
Kirti