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

[Alex Williamson <alex.williamson@redhat.com>]

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,

Alex