Re: [virtio-comment] [PATCH V2 2/2] virtio: introduce STOP status bit

[Stefan Hajnoczi <stefanha@redhat.com>]

On Wed, Jul 21, 2021 at 10:52:15AM +0800, Jason Wang wrote:
> 
> å 2021/7/20 äå6:19, Stefan Hajnoczi åé:
> > On Tue, Jul 20, 2021 at 11:02:42AM +0800, Jason Wang wrote:
> > > å 2021/7/19 äå8:43, Stefan Hajnoczi åé:
> > > > On Fri, Jul 16, 2021 at 10:03:17AM +0800, Jason Wang wrote:
> > > > > å 2021/7/15 äå6:01, Stefan Hajnoczi åé:
> > > > > > On Thu, Jul 15, 2021 at 09:35:13AM +0800, Jason Wang wrote:
> > > > > > > å 2021/7/14 äå11:07, Stefan Hajnoczi åé:
> > > > > > > > On Wed, Jul 14, 2021 at 06:29:28PM +0800, Jason Wang wrote:
> > > > > > > > > å 2021/7/14 äå5:53, Stefan Hajnoczi åé:
> > > > > > > > > > On Tue, Jul 13, 2021 at 08:16:35PM +0800, Jason Wang wrote:
> > > > > > > > > > > å 2021/7/13 äå6:00, Stefan Hajnoczi åé:
> > > > > > > > > > > > On Tue, Jul 13, 2021 at 11:27:03AM +0800, Jason Wang wrote:
> > > > > > > > > > > > > å 2021/7/12 äå5:57, Stefan Hajnoczi åé:
> > > > > > > > > > > > > > On Mon, Jul 12, 2021 at 12:00:39PM +0800, Jason Wang wrote:
> > > > > > > > > > > > > > > å 2021/7/11 äå4:36, Michael S. Tsirkin åé:
> > > > > > > > > > > > > > > > On Fri, Jul 09, 2021 at 07:23:33PM +0200, Eugenio Perez Martin wrote:
> > > > > > > > > > > > > > > > > > >           If I understand correctly, this is all
> > > > > > > > > > > > > > > > > > > driven from the driver inside the guest, so for this to work
> > > > > > > > > > > > > > > > > > > the guest must be running and already have initialised the driver.
> > > > > > > > > > > > > > > > > > Yes.
> > > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > As I see it, the feature can be driven entirely by the VMM as long as
> > > > > > > > > > > > > > > > > it intercept the relevant configuration space (PCI, MMIO, etc) from
> > > > > > > > > > > > > > > > > guest's reads and writes, and present it as coherent and transparent
> > > > > > > > > > > > > > > > > for the guest. Some use cases I can imagine with a physical device (or
> > > > > > > > > > > > > > > > > vp_vpda device) with VIRTIO_F_STOP:
> > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > 1) The VMM chooses not to pass the feature flag. The guest cannot stop
> > > > > > > > > > > > > > > > > the device, so any write to this flag is an error/undefined.
> > > > > > > > > > > > > > > > > 2) The VMM passes the flag to the guest. The guest can stop the device.
> > > > > > > > > > > > > > > > > 2.1) The VMM stops the device to perform a live migration, and the
> > > > > > > > > > > > > > > > > guest does not write to STOP in any moment of the LM. It resets the
> > > > > > > > > > > > > > > > > destination device with the state, and then initializes the device.
> > > > > > > > > > > > > > > > > 2.2) The guest stops the device and, when STOP(32) is set, the source
> > > > > > > > > > > > > > > > > VMM migrates the device status. The destination VMM realizes the bit,
> > > > > > > > > > > > > > > > > so it sets the bit in the destination too after device initialization.
> > > > > > > > > > > > > > > > > 2.3) The device is not initialized by the guest so it doesn't matter
> > > > > > > > > > > > > > > > > what bit has the HW, but the VM can be migrated.
> > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > Am I missing something?
> > > > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > > Thanks!
> > > > > > > > > > > > > > > > It's doable like this. It's all a lot of hoops to jump through though.
> > > > > > > > > > > > > > > > It's also not easy for devices to implement.
> > > > > > > > > > > > > > > It just requires a new status bit. Anything that makes you think it's hard
> > > > > > > > > > > > > > > to implement?
> > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > E.g for networking device, it should be sufficient to use this bit + the
> > > > > > > > > > > > > > > virtqueue state.
> > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > > Why don't we design the feature in a way that is useable by VMMs
> > > > > > > > > > > > > > > > and implementable by devices in a simple way?
> > > > > > > > > > > > > > > It use the common technology like register shadowing without any further
> > > > > > > > > > > > > > > stuffs.
> > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > Or do you have any other ideas?
> > > > > > > > > > > > > > > 
> > > > > > > > > > > > > > > (I think we all know migration will be very hard if we simply pass through
> > > > > > > > > > > > > > > those state registers).
> > > > > > > > > > > > > > If an admin virtqueue is used instead of the STOP Device Status field
> > > > > > > > > > > > > > bit then there's no need to re-read the Device Status field in a loop
> > > > > > > > > > > > > > until the device has stopped.
> > > > > > > > > > > > > Probably not. Let me to clarify several points:
> > > > > > > > > > > > > 
> > > > > > > > > > > > > - This proposal has nothing to do with admin virtqueue. Actually, admin
> > > > > > > > > > > > > virtqueue could be used for carrying any basic device facility like status
> > > > > > > > > > > > > bit. E.g I'm going to post patches that use admin virtqueue as a "transport"
> > > > > > > > > > > > > for device slicing at virtio level.
> > > > > > > > > > > > > - Even if we had introduced admin virtqueue, we still need a per function
> > > > > > > > > > > > > interface for this. This is a must for nested virtualization, we can't
> > > > > > > > > > > > > always expect things like PF can be assigned to L1 guest.
> > > > > > > > > > > > > - According to the proposal, there's no need for the device to complete all
> > > > > > > > > > > > > the consumed buffers, device can choose to expose those inflight descriptors
> > > > > > > > > > > > > in a device specific way and set the STOP bit. This means, if we have the
> > > > > > > > > > > > > device specific in-flight descriptor reporting facility, the device can
> > > > > > > > > > > > > almost set the STOP bit immediately.
> > > > > > > > > > > > > - If we don't go with the basic device facility but using the admin
> > > > > > > > > > > > > virtqueue specific method, we still need to clarify how it works with the
> > > > > > > > > > > > > device status state machine, it will be some kind of sub-states which looks
> > > > > > > > > > > > > much more complicated than the current proposal.
> > > > > > > > > > > > > 
> > > > > > > > > > > > > 
> > > > > > > > > > > > > > When migrating a guest with many VIRTIO devices a busy waiting approach
> > > > > > > > > > > > > > extends downtime if implemented sequentially (stopping one device at a
> > > > > > > > > > > > > > time).
> > > > > > > > > > > > > Well. You need some kinds of waiting for sure, the device/DMA needs sometime
> > > > > > > > > > > > > to be stopped. The downtime is determined by a specific virtio
> > > > > > > > > > > > > implementation which is hard to be restricted at the spec level. We can
> > > > > > > > > > > > > clarify that the device must set the STOP bit in e.g 100ms.
> > > > > > > > > > > > > 
> > > > > > > > > > > > > 
> > > > > > > > > > > > > >         It can be implemented concurrently (setting the STOP bit on all
> > > > > > > > > > > > > > devices and then looping until all their Device Status fields have the
> > > > > > > > > > > > > > bit set), but this becomes more complex to implement.
> > > > > > > > > > > > > I still don't get what kind of complexity did you worry here.
> > > > > > > > > > > > > 
> > > > > > > > > > > > > 
> > > > > > > > > > > > > > I'm a little worried about adding a new bit that requires busy
> > > > > > > > > > > > > > waiting...
> > > > > > > > > > > > > Busy wait is not something that is introduced in this patch:
> > > > > > > > > > > > > 
> > > > > > > > > > > > > 4.1.4.3.2 Driver Requirements: Common configuration structure layout
> > > > > > > > > > > > > 
> > > > > > > > > > > > > After writing 0 to device_status, the driver MUST wait for a read of
> > > > > > > > > > > > > device_status to return 0 before reinitializing the device.
> > > > > > > > > > > > > 
> > > > > > > > > > > > > Since it was required for at least one transport. We need do something
> > > > > > > > > > > > > similar to when introducing basic facility.
> > > > > > > > > > > > Adding the STOP but as a Device Status bit is a small and clean VIRTIO
> > > > > > > > > > > > spec change. I like that.
> > > > > > > > > > > > 
> > > > > > > > > > > > On the other hand, devices need time to stop and that time can be
> > > > > > > > > > > > unbounded. For example, software virtio-blk/scsi implementations since
> > > > > > > > > > > > cannot immediately cancel in-flight I/O requests on Linux hosts.
> > > > > > > > > > > > 
> > > > > > > > > > > > The natural interface for long-running operations is virtqueue requests.
> > > > > > > > > > > > That's why I mentioned the alternative of using an admin virtqueue
> > > > > > > > > > > > instead of a Device Status bit.
> > > > > > > > > > > So I'm not against the admin virtqueue. As said before, admin virtqueue
> > > > > > > > > > > could be used for carrying the device status bit.
> > > > > > > > > > > 
> > > > > > > > > > > Send a command to set STOP status bit to admin virtqueue. Device will make
> > > > > > > > > > > the command buffer used after it has successfully stopped the device.
> > > > > > > > > > > 
> > > > > > > > > > > AFAIK, they are not mutually exclusive, since they are trying to solve
> > > > > > > > > > > different problems.
> > > > > > > > > > > 
> > > > > > > > > > > Device status - basic device facility
> > > > > > > > > > > 
> > > > > > > > > > > Admin virtqueue - transport/device specific way to implement (part of) the
> > > > > > > > > > > device facility
> > > > > > > > > > > 
> > > > > > > > > > > > Although you mentioned that the stopped state needs to be reflected in
> > > > > > > > > > > > the Device Status field somehow, I'm not sure about that since the
> > > > > > > > > > > > driver typically doesn't need to know whether the device is being
> > > > > > > > > > > > migrated.
> > > > > > > > > > > The guest won't see the real device status bit. VMM will shadow the device
> > > > > > > > > > > status bit in this case.
> > > > > > > > > > > 
> > > > > > > > > > > E.g with the current vhost-vDPA, vDPA behave like a vhost device, guest is
> > > > > > > > > > > unaware of the migration.
> > > > > > > > > > > 
> > > > > > > > > > > STOP status bit is set by Qemu to real virtio hardware. But guest will only
> > > > > > > > > > > see the DRIVER_OK without STOP.
> > > > > > > > > > > 
> > > > > > > > > > > It's not hard to implement the nested on top, see the discussion initiated
> > > > > > > > > > > by Eugenio about how expose VIRTIO_F_STOP to guest for nested live
> > > > > > > > > > > migration.
> > > > > > > > > > > 
> > > > > > > > > > > 
> > > > > > > > > > > >        In fact, the VMM would need to hide this bit and it's safer to
> > > > > > > > > > > > keep it out-of-band instead of risking exposing it by accident.
> > > > > > > > > > > See above, VMM may choose to hide or expose the capability. It's useful for
> > > > > > > > > > > migrating a nested guest.
> > > > > > > > > > > 
> > > > > > > > > > > If we design an interface that can be used in the nested environment, it's
> > > > > > > > > > > not an ideal interface.
> > > > > > > > > > > 
> > > > > > > > > > > 
> > > > > > > > > > > > In addition, stateful devices need to load/save non-trivial amounts of
> > > > > > > > > > > > data. They need DMA to do this efficiently, so an admin virtqueue is a
> > > > > > > > > > > > good fit again.
> > > > > > > > > > > I don't get the point here. You still need to address the exact the similar
> > > > > > > > > > > issues for admin virtqueue: the unbound time in freezing the device, the
> > > > > > > > > > > interaction with the virtio device status state machine.
> > > > > > > > > > Device state state can be large so a register interface would be a
> > > > > > > > > > bottleneck. DMA is needed. I think a virtqueue is a good fit for
> > > > > > > > > > saving/loading device state.
> > > > > > > > > So this patch doesn't mandate a register interface, isn't it?
> > > > > > > > You're right, not this patch. I mentioned it because your other patch
> > > > > > > > series ("[PATCH] virtio-pci: implement VIRTIO_F_RING_STATE") implements
> > > > > > > > it a register interface.
> > > > > > > > 
> > > > > > > > > And DMA
> > > > > > > > > doesn't means a virtqueue, it could be a transport specific method.
> > > > > > > > Yes, although virtqueues are a pretty good interface that works across
> > > > > > > > transports (PCI/MMIO/etc) thanks to the standard vring memory layout.
> > > > > > > > 
> > > > > > > > > I think we need to start from defining the state of one specific device and
> > > > > > > > > see what is the best interface.
> > > > > > > > virtio-blk might be the simplest. I think virtio-net has more device
> > > > > > > > state and virtio-scsi is definitely more complext than virtio-blk.
> > > > > > > > 
> > > > > > > > First we need agreement on whether "device state" encompasses the full
> > > > > > > > state of the device or just state that is unknown to the VMM.
> > > > > > > I think we've discussed this in the past. It can't work since:
> > > > > > > 
> > > > > > > 1) The state and its format must be clearly defined in the spec
> > > > > > > 2) We need to maintain migration compatibility and debug-ability
> > > > > > Some devices need implementation-specific state. They should still be
> > > > > > able to live migrate even if it means cross-implementation migration and
> > > > > > debug-ability is not possible.
> > > > > I think we need to re-visit this conclusion. Migration compatibility is
> > > > > pretty important, especially consider the software stack has spent a huge
> > > > > mount of effort in maintaining them.
> > > > > 
> > > > > Say a virtio hardware would break this, this mean we will lose all the
> > > > > advantages of being a standard device.
> > > > > 
> > > > > If we can't do live migration among:
> > > > > 
> > > > > 1) different backends, e.g migrate from virtio hardware to migrate software
> > > > > 2) different vendors
> > > > > 
> > > > > We failed to say as a standard device and the customer is in fact locked by
> > > > > the vendor implicitly.
> > > > My virtiofs device implementation is backed by an in-memory file system.
> > > > The device state includes the contents of each file.
> > > > 
> > > > Your virtiofs device implementation uses Linux file handles to keep
> > > > track of open files. The device state includes Linux file handles (but
> > > > not the contents of each file) so the destination host can access the
> > > > same files on shared storage.
> > > > 
> > > > Cornelia's virtiofs device implementation is backed by an object storage
> > > > HTTP API. The device state includes API object IDs.
> > > > 
> > > > The device state is implementation-dependent. There is no standard
> > > > representation and it's not possible to migrate between device
> > > > implementations. How are they supposed to migrate?
> > > 
> > > So if I understand correclty, virtio-fs is not desigined to be migrate-able?
> > > 
> > > (Having a check on the current virtio-fs support in qemu, it looks to me it
> > > has a migration blocker).
> > The code does not support live migration but it's on the roadmap. Max
> > Reitz added Linux file handle support to virtiofsd. That was the first
> > step towards being able to migrate the device's state.
> 
> 
> A dumb question, how do qemu know it is connected to virtiofsd?

virtiofsd is a vhost-user-fs device. QEMU doesn't know if it's connected
to virtiofsd or another implementation.

> > > > This is why I think it's necessarily to allow implementation-specific
> > > > device state representations.
> > > 
> > > Or you probably mean you don't support cross backend migration. This sounds
> > > like a drawback and it's actually not a standard device but a
> > > vendor/implementation specific device.
> > > 
> > > It would bring a lot of troubles, not only for the implementation but for
> > > the management. Maybe we can start from adding the support of migration for
> > > some specific backend and start from there.
> > Yes, it's complicated. Some implementations could be compatible, but
> > others can never be compatible because they have completely different
> > state.
> > 
> > The virtiofsd implementation is the main one for virtiofs and the device
> > state representation can be published, even standardized. Others can
> > implement it to achieve migration compatibility.
> > 
> > But it must be possible for implementations that have completely
> > different state to migrate too. virtiofsd isn't special.
> > 
> > > > > > > 3) Not a proper uAPI desgin
> > > > > > I never understood this argument. The Linux uAPI passes through lots of
> > > > > > opaque data from devices to userspace. Allowing an
> > > > > > implementation-specific device state representation is nothing new. VFIO
> > > > > > already does it.
> > > > > I think we've already had a lots of discussion for VFIO but without a
> > > > > conclusion. Maybe we need the verdict from Linus or Greg (not sure if it's
> > > > > too late). But that's not related to virito and this thread.
> > > > > 
> > > > > What you propose here is kind of conflict with the efforts of virtio. I
> > > > > think we all aggree that we should define the state in the spec. Assuming
> > > > > this is correct:
> > > > > 
> > > > > 1) why do we still offer opaque migration state to userspace?
> > > > See above. Stateful devices may require an implementation-defined device
> > > > state representation.
> > > 
> > > So my point stand still, it's not a standard device if we do this.
> > These "non-standard devices" still need to be able to migrate.
> 
> 
> See other thread, it breaks the effort of having a spec.
>
> >   How
> > should we do that?
> 
> 
> I think the main issue is that, to me it's not a virtio device but a device
> that is using virtio queue with implementation specific state. So it can't
> be migrated by the virtio subsystem but through a vendor/implementation
> specific migration driver.

Okay. Are you thinking about a separate set of vDPA APIs and vhost
ioctls so the VMM can save/load implementation-specific device state?
These separate APIs just need to be called as part of the standard
VIRTIO stop and vq save/load lifecycle.

> > 
> > > > > 2) how can it be integrated into the current VMM (Qemu) virtio devices'
> > > > > migration bytes stream?
> > > > Opaque data like D-Bus VMState:
> > > > https://qemu.readthedocs.io/en/latest/interop/dbus-vmstate.html
> > > 
> > > Actually, I meant how to keep the opaque state which is compatible with all
> > > the existing device that can do migration.
> > > 
> > > E.g we want to live migration virtio-blk among any backends (from a hardware
> > > device to a software backend).
> > There was a series of email threads last year where migration
> > compatibility was discussed:
> > 
> > https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg02620.html
> > 
> > I proposed an algorithm for checking migration compatibility between
> > devices. The source and destination device can have different
> > implementations (e.g. hardware, software, etc).
> > 
> > It involves picking an identifier like virtio-spec.org/pci/virtio-net
> > for the device state representation and device parameters for aspects of
> > the device that vary between instances (e.g. tso=on|off).
> > 
> > It's more complex than today's live migration approach in libvirt and
> > QEMU. Today libvirt configures the source and destination in a
> > compatible manner (thanks to knowledge of the device implementation) and
> > then QEMU transfers the device state.
> > 
> > Part of the point of defining a migration compatibility algorithm is
> > that it's possible to lift the assumptions out of libvirt so that
> > arbitrary device implementations can be supported (hardware, software,
> > etc) without putting knowledge about every device/VMM implementation
> > into libvirt.
> > 
> > (The other advantage is that this allows orchestration software to
> > determine migration compatibility before starting a migration.)
> 
> 
> This looks like another independent issues and I fully agree to have a
> better migration protocol. But using that means we break the migration
> compatibility with the existing device which is used for more than 10 years.
> We still need to make the migration from/to the existing virtio device to
> work.

I agree that migrating to/from existing devices needs to work. It should
be possible to transition without breaking migration.

> > > > > > > > That's
> > > > > > > > basically the difference between the vhost/vDPA's selective passthrough
> > > > > > > > approach and VFIO's full passthrough approach.
> > > > > > > We can't do VFIO full pasthrough for migration anyway, some kind of mdev is
> > > > > > > required but it's duplicated with the current vp_vdpa driver.
> > > > > > I'm not sure that's true. Generic VFIO PCI migration can probably be
> > > > > > achieved without mdev:
> > > > > > 1. Define a migration PCI Capability that indicates support for
> > > > > >       VFIO_REGION_TYPE_MIGRATION. This allows the PCI device to implement
> > > > > >       the migration interface in hardware instead of an mdev driver.
> > > > > So I think it still depend on the driver to implement migrate state which is
> > > > > vendor specific.
> > > > The current VFIO migration interface depends on a device-specific
> > > > software mdev driver but here I'm showing that the physical device can
> > > > implement the migration interface so that no device-specific driver code
> > > > is needed.
> > > 
> > > This is not what I read from the patch:
> > > 
> > >  Â* device_state: (read/write)
> > >  Â*ÂÂÂÂÂ - The user application writes to this field to inform the vendor
> > > driver
> > >  Â*ÂÂÂÂÂÂÂ about the device state to be transitioned to.
> > >  Â*ÂÂÂÂÂ - The vendor driver should take the necessary actions to change the
> > >  Â*ÂÂÂÂÂÂÂ device state. After successful transition to a given state, the
> > >  Â*ÂÂÂÂÂÂÂ vendor driver should return success on write(device_state, state)
> > >  Â*ÂÂÂÂÂÂÂ system call. If the device state transition fails, the vendor
> > > driver
> > >  Â*ÂÂÂÂÂÂÂ should return an appropriate -errno for the fault condition.
> > > 
> > > Vendor driver need to mediate between the uAPI and the actual device.
> > Yes, that's the current state of VFIO migration. If a hardware interface
> > (e.g. PCI Capability) is defined that maps to this API then no
> > device-specific drivers would be necessary because core VFIO PCI code
> > can implement the uAPI by talking to the hardware.
> 
> 
> As we discussed, it would be very hard. The device state is implementation
> specific which may not fit for the Capability. (PCIE has already had VF
> migration state in the SR-IOV extended capability).
> 
> 
> > 
> > > > > > 2. The VMM either uses the migration PCI Capability directly from
> > > > > >       userspace or core VFIO PCI code advertises VFIO_REGION_TYPE_MIGRATION
> > > > > >       to userspace so migration can proceed in the same way as with
> > > > > >       VFIO/mdev drivers.
> > > > > > 3. The PCI Capability is not passed through to the guest.
> > > > > This brings troubles in the nested environment.
> > > > It depends on the device splitting/management design. If L0 wishes to
> > > > let L1 manage the VFs then it would need to expose a management device.
> > > > Since the migration interface is generic (not device-specific) a generic
> > > > management device solves this for all devices.
> > > 
> > > Right, but it's a burden to expose the management device or it may just
> > > won't work.
> > A single generic management device is not a huge burden and it may turn
> > out that keeping the management device out-of-band is actually a
> > desirable feature if the device owner does not wish to expose the
> > stop/save/load functionality for some reason.
> 
> 
> VMM are free to hide those features from guest. Management can just do
> -device virtio-pci,state=false
> 
> Having management device works for L0 but not suitable for L(x>0). A per
> function device interface is a must for the nested virt to work in a simple
> and easy way.

You are right, a per function interface is simplest. I'm not experienced
enough with SR-IOV and nested virtualization to have a strong opinion in
this area.

Stefan

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