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

[Dr. David Alan Gilbert]

* Alex Williamson (address@hidden) wrote:
> On Thu, 2 Jan 2020 18:25:37 +0000
> "Dr. David Alan Gilbert" <address@hidden> wrote:
> 
> > * Alex Williamson (address@hidden) wrote:
> > > On Fri, 20 Dec 2019 01:40:35 +0530
> > > Kirti Wankhede <address@hidden> 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.

OK - but that doesn't give you any way to figure out 'why' it failed;
I guess I was expecting you to then read an 'error' register to find
out what happened.
Assuming the write() to transition to stop-and-copy fails and you're
still in pre-copy, what's the defined thing you're supposed to do next?
Decide migration has failed and then do a write() to transition to running?

> > 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,
> 
o
Dave
> Alex
--
Dr. David Alan Gilbert / address@hidden / Manchester, UK