On Mon, 25 Oct 2021 17:34:01 +0100 "Dr. David Alan Gilbert" <dgilbert@xxxxxxxxxx> wrote: > * Alex Williamson (alex.williamson@xxxxxxxxxx) wrote: > > [Cc +dgilbert, +cohuck] > > > > On Wed, 20 Oct 2021 11:28:04 +0300 > > Yishai Hadas <yishaih@xxxxxxxxxx> wrote: > > > > > On 10/20/2021 2:04 AM, Jason Gunthorpe wrote: > > > > On Tue, Oct 19, 2021 at 02:58:56PM -0600, Alex Williamson wrote: > > > >> I think that gives us this table: > > > >> > > > >> | NDMA | RESUMING | SAVING | RUNNING | > > > >> +----------+----------+----------+----------+ --- > > > >> | X | 0 | 0 | 0 | ^ > > > >> +----------+----------+----------+----------+ | > > > >> | 0 | 0 | 0 | 1 | | > > > >> +----------+----------+----------+----------+ | > > > >> | X | 0 | 1 | 0 | > > > >> +----------+----------+----------+----------+ NDMA value is either compatible > > > >> | 0 | 0 | 1 | 1 | to existing behavior or don't > > > >> +----------+----------+----------+----------+ care due to redundancy vs > > > >> | X | 1 | 0 | 0 | !_RUNNING/INVALID/ERROR > > > >> +----------+----------+----------+----------+ > > > >> | X | 1 | 0 | 1 | | > > > >> +----------+----------+----------+----------+ | > > > >> | X | 1 | 1 | 0 | | > > > >> +----------+----------+----------+----------+ | > > > >> | X | 1 | 1 | 1 | v > > > >> +----------+----------+----------+----------+ --- > > > >> | 1 | 0 | 0 | 1 | ^ > > > >> +----------+----------+----------+----------+ Desired new useful cases > > > >> | 1 | 0 | 1 | 1 | v > > > >> +----------+----------+----------+----------+ --- > > > >> > > > >> Specifically, rows 1, 3, 5 with NDMA = 1 are valid states a user can > > > >> set which are simply redundant to the NDMA = 0 cases. > > > > It seems right > > > > > > > >> Row 6 remains invalid due to lack of support for pre-copy (_RESUMING > > > >> | _RUNNING) and therefore cannot be set by userspace. Rows 7 & 8 > > > >> are error states and cannot be set by userspace. > > > > I wonder, did Yishai's series capture this row 6 restriction? Yishai? > > > > > > > > > It seems so, by using the below check which includes the > > > !VFIO_DEVICE_STATE_VALID clause. > > > > > > if (old_state == VFIO_DEVICE_STATE_ERROR || > > > !VFIO_DEVICE_STATE_VALID(state) || > > > (state & ~MLX5VF_SUPPORTED_DEVICE_STATES)) > > > return -EINVAL; > > > > > > Which is: > > > > > > #define VFIO_DEVICE_STATE_VALID(state) \ > > > (state & VFIO_DEVICE_STATE_RESUMING ? \ > > > (state & VFIO_DEVICE_STATE_MASK) == VFIO_DEVICE_STATE_RESUMING : 1) > > > > > > > > > > >> Like other bits, setting the bit should be effective at the completion > > > >> of writing device state. Therefore the device would need to flush any > > > >> outbound DMA queues before returning. > > > > Yes, the device commands are expected to achieve this. > > > > > > > >> The question I was really trying to get to though is whether we have a > > > >> supportable interface without such an extension. There's currently > > > >> only an experimental version of vfio migration support for PCI devices > > > >> in QEMU (afaik), > > > > If I recall this only matters if you have a VM that is causing > > > > migratable devices to interact with each other. So long as the devices > > > > are only interacting with the CPU this extra step is not strictly > > > > needed. > > > > > > > > So, single device cases can be fine as-is > > > > > > > > IMHO the multi-device case the VMM should probably demand this support > > > > from the migration drivers, otherwise it cannot know if it is safe for > > > > sure. > > > > > > > > A config option to override the block if the admin knows there is no > > > > use case to cause devices to interact - eg two NVMe devices without > > > > CMB do not have a useful interaction. > > > > > > > >> so it seems like we could make use of the bus-master bit to fill > > > >> this gap in QEMU currently, before we claim non-experimental > > > >> support, but this new device agnostic extension would be required > > > >> for non-PCI device support (and PCI support should adopt it as > > > >> available). Does that sound right? Thanks, > > > > I don't think the bus master support is really a substitute, tripping > > > > bus master will stop DMA but it will not do so in a clean way and is > > > > likely to be non-transparent to the VM's driver. > > > > > > > > The single-device-assigned case is a cleaner restriction, IMHO. > > > > > > > > Alternatively we can add the 4th bit and insist that migration drivers > > > > support all the states. I'm just unsure what other HW can do, I get > > > > the feeling people have been designing to the migration description in > > > > the header file for a while and this is a new idea. > > > > I'm wondering if we're imposing extra requirements on the !_RUNNING > > state that don't need to be there. For example, if we can assume that > > all devices within a userspace context are !_RUNNING before any of the > > devices begin to retrieve final state, then clearing of the _RUNNING > > bit becomes the device quiesce point and the beginning of reading > > device data is the point at which the device state is frozen and > > serialized. No new states required and essentially works with a slight > > rearrangement of the callbacks in this series. Why can't we do that? > > So without me actually understanding your bit encodings that closely, I > think the problem is we have to asusme that any transition takes time. > From the QEMU point of view I think the requirement is when we stop the > machine (vm_stop_force_state(RUN_STATE_FINISH_MIGRATE) in > migration_completion) that at the point that call returns (with no > error) all devices are idle. That means you need a way to command the > device to go into the stopped state, and probably another to make sure > it's got there. In a way. We're essentially recognizing that we cannot stop a single device in isolation of others that might participate in peer-to-peer DMA with that device, so we need to make a pass to quiesce each device before we can ask the device to fully stop. This new device state bit is meant to be that quiescent point, devices can accept incoming DMA but should cease to generate any. Once all device are quiesced then we can safely stop them. > Now, you could be a *little* more sloppy; you could allow a device carry > on doing stuff purely with it's own internal state up until the point > it needs to serialise; but that would have to be strictly internal state > only - if it can change any other devices state (or issue an interrupt, > change RAM etc) then you get into ordering issues on the serialisation > of multiple devices. Yep, that's the proposal that doesn't require a uAPI change, we loosen the definition of stopped to mean the device can no longer generate DMA or interrupts and all internal processing outside or responding to incoming DMA should halt (essentially the same as the new quiescent state above). Once all devices are in this state, there should be no incoming DMA and we can safely collect per device migration data. If state changes occur beyond the point in time where userspace has initiated the collection of migration data, drivers have options for generating errors when userspace consumes that data. AFAICT, the two approaches are equally valid. If we modify the uAPI to include this new quiescent state then userspace needs to make some hard choices about what configurations they support without such a feature. The majority of configurations are likely not exercising p2p between assigned devices, but the hypervisor can't know that. If we work within the existing uAPI, well there aren't any open source driver implementations yet anyway and any non-upstream implementations would need to be updated for this clarification. Existing userspace works better with no change, so long as they already follow the guideline that all devices in the userspace context must be stopped before the migration data of any device can be considered valid. Thanks, Alex
