On Tue, 7 Jan 2020 12:58:22 +0530 Kirti Wankhede <kwankhede@xxxxxxxxxx> wrote: > On 1/7/2020 4:48 AM, Alex Williamson 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. > > > >> 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, > > > > Instead of using invalid state by vendor driver on device failure, I > think better to reserve one bit in device state which vendor driver can > set on device failure. When error bit is set, other bits in device state > should be ignored. Why is a separate bit better? Saving and Restoring states are mutually exclusive, so we have an unused and invalid device state already without burning another bit. Thanks, Alex
