Messages in this thread | | | Subject | Re: [PATCH mlx5-next 2/7] vfio: Add an API to check migration state transition validity | From | Max Gurtovoy <> | Date | Wed, 29 Sep 2021 13:57:13 +0300 |
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On 9/28/2021 10:19 PM, Alex Williamson wrote: > On Mon, 27 Sep 2021 20:12:39 -0300 > Jason Gunthorpe <jgg@ziepe.ca> wrote: > >> On Mon, Sep 27, 2021 at 04:46:48PM -0600, Alex Williamson wrote: >>>> + enum { MAX_STATE = VFIO_DEVICE_STATE_RESUMING }; >>>> + static const u8 vfio_from_state_table[MAX_STATE + 1][MAX_STATE + 1] = { >>>> + [VFIO_DEVICE_STATE_STOP] = { >>>> + [VFIO_DEVICE_STATE_RUNNING] = 1, >>>> + [VFIO_DEVICE_STATE_RESUMING] = 1, >>>> + }, >>> Our state transition diagram is pretty weak on reachable transitions >>> out of the _STOP state, why do we select only these two as valid? >> I have no particular opinion on specific states here, however adding >> more states means more stuff for drivers to implement and more risk >> driver writers will mess up this uAPI. > It looks like state transitions were largely discussed in v9 and v10 of > the migration proposals: > > https://lore.kernel.org/all/1573578220-7530-2-git-send-email-kwankhede@nvidia.com/ > https://lore.kernel.org/all/1576527700-21805-2-git-send-email-kwankhede@nvidia.com/ > > I'm not seeing that we really excluded many transitions there. > >> So only on those grounds I'd suggest to keep this to the minimum >> needed instead of the maximum logically possible.. >> >> Also, probably the FSM comment from the uapi header file should be >> moved into a function comment above this function? > It's not clear this function shouldn't be anything more than: > > if (new_state > MAX_STATE || old_state > MAX_STATE) > return false; /* exited via device reset, */ > /* entered via transition fault */ > > return true; > > That's still only 5 fully interconnected states to work between, and > potentially a 6th if we decide _RESUMING|_RUNNING is valid for a device > supporting post-copy. > > In defining the device state, we tried to steer away from defining it > in terms of the QEMU migration API, but rather as a set of controls > that could be used to support that API to leave us some degree of > independence that QEMU implementation might evolve.
The state machine is not related to QEMU specifically.
The state machine defines an agreement between user application (let's say QEMU) and VFIO.
If a user application would like to move, for example, from RESUMING to SAVING state, then the kernel should fail. I don't that there is a device that can support it.
If you prefer we check this inside our mlx5 vfio driver, we can do it. But we think that this is a common logic according to the defined FSM.
Do you prefer code duplication in vendor vfio-pci drivers ?
> To that extent, it actually seems easier for a device implementation to > focus on bit definition rather than the state machine node. > > I'd also vote that any clarification of state validity and transitions > belongs in the uAPI header and a transition test function should > reference that header as the source of truth, rather than the other way > around. Thanks,
Yes, I guess this is possible.
> > Alex >
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