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

[Dr. David Alan Gilbert]

* Alex Williamson (address@hidden) wrote:
> On Tue, 7 Jan 2020 09:57:40 +0000
> "Dr. David Alan Gilbert" <address@hidden> wrote:
> 
> > * 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?
> 
> 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 / address@hidden / Manchester, UK