* Alex Williamson (alex.williamson@xxxxxxxxxx) wrote: > On Tue, 7 Jan 2020 09:57:40 +0000 > "Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx> wrote: > > > * 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? > > Defining semantics for an error register seems like a project on its > own. We do have flags, we could use them to add an error register > later, but I think it's only going to rat hole this effort to try to > incorporate that now. OK, to be honest I didn't really mean for that thing to be used by code to decide on it's next action, rather to have something to report when it failed. > The state machine is fairly small, so in the > scenario you present, I think the user would assume a failure at > pre-copy to stop-and-copy transition would fail the migration and the > device could go back to running state. If the device then fails to > return to the running state, we might be stuck with a device with > reduced performance or overhead and the user could warn about that and > continue with the device as-is. The vendor drivers could make use of > -EAGAIN on transition failure to indicate a temporary issue, but > otherwise the user should probably consider it a persistent error until > either a device reset or start of a new migration sequence (ie. return > to running and start over). Thanks, OK as long as we define somewhere that the action on a failed transition is then try and transitino to running before restarting the VM and fail the migration. Dave > Alex -- Dr. David Alan Gilbert / dgilbert@xxxxxxxxxx / Manchester, UK
