* Alex Williamson (alex.williamson@xxxxxxxxxx) wrote: > 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. 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 / dgilbert@xxxxxxxxxx / Manchester, UK
