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