Re: [PATCH 2/4] vhost-user: Interface for migration state transfer

[Stefan Hajnoczi]

On Thu, 13 Apr 2023 at 13:55, Hanna Czenczek <hreitz@redhat.com> wrote:
>
> On 13.04.23 13:38, Stefan Hajnoczi wrote:
> > On Thu, 13 Apr 2023 at 05:24, Hanna Czenczek <hreitz@redhat.com> wrote:
> >> On 12.04.23 23:06, Stefan Hajnoczi wrote:
> >>> On Tue, Apr 11, 2023 at 05:05:13PM +0200, Hanna Czenczek wrote:
> >>>> So-called "internal" virtio-fs migration refers to transporting the
> >>>> back-end's (virtiofsd's) state through qemu's migration stream.  To do
> >>>> this, we need to be able to transfer virtiofsd's internal state to and
> >>>> from virtiofsd.
> >>>>
> >>>> Because virtiofsd's internal state will not be too large, we believe it
> >>>> is best to transfer it as a single binary blob after the streaming
> >>>> phase.  Because this method should be useful to other vhost-user
> >>>> implementations, too, it is introduced as a general-purpose addition to
> >>>> the protocol, not limited to vhost-user-fs.
> >>>>
> >>>> These are the additions to the protocol:
> >>>> - New vhost-user protocol feature VHOST_USER_PROTOCOL_F_MIGRATORY_STATE:
> >>>>     This feature signals support for transferring state, and is added so
> >>>>     that migration can fail early when the back-end has no support.
> >>>>
> >>>> - SET_DEVICE_STATE_FD function: Front-end and back-end negotiate a pipe
> >>>>     over which to transfer the state.  The front-end sends an FD to the
> >>>>     back-end into/from which it can write/read its state, and the 
> >>>> back-end
> >>>>     can decide to either use it, or reply with a different FD for the
> >>>>     front-end to override the front-end's choice.
> >>>>     The front-end creates a simple pipe to transfer the state, but maybe
> >>>>     the back-end already has an FD into/from which it has to write/read
> >>>>     its state, in which case it will want to override the simple pipe.
> >>>>     Conversely, maybe in the future we find a way to have the front-end
> >>>>     get an immediate FD for the migration stream (in some cases), in 
> >>>> which
> >>>>     case we will want to send this to the back-end instead of creating a
> >>>>     pipe.
> >>>>     Hence the negotiation: If one side has a better idea than a plain
> >>>>     pipe, we will want to use that.
> >>>>
> >>>> - CHECK_DEVICE_STATE: After the state has been transferred through the
> >>>>     pipe (the end indicated by EOF), the front-end invokes this function
> >>>>     to verify success.  There is no in-band way (through the pipe) to
> >>>>     indicate failure, so we need to check explicitly.
> >>>>
> >>>> Once the transfer pipe has been established via SET_DEVICE_STATE_FD
> >>>> (which includes establishing the direction of transfer and migration
> >>>> phase), the sending side writes its data into the pipe, and the reading
> >>>> side reads it until it sees an EOF.  Then, the front-end will check for
> >>>> success via CHECK_DEVICE_STATE, which on the destination side includes
> >>>> checking for integrity (i.e. errors during deserialization).
> >>>>
> >>>> Suggested-by: Stefan Hajnoczi <stefanha@redhat.com>
> >>>> Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
> >>>> ---
> >>>>    include/hw/virtio/vhost-backend.h |  24 +++++
> >>>>    include/hw/virtio/vhost.h         |  79 ++++++++++++++++
> >>>>    hw/virtio/vhost-user.c            | 147 ++++++++++++++++++++++++++++++
> >>>>    hw/virtio/vhost.c                 |  37 ++++++++
> >>>>    4 files changed, 287 insertions(+)
> >>>>
> >>>> diff --git a/include/hw/virtio/vhost-backend.h 
> >>>> b/include/hw/virtio/vhost-backend.h
> >>>> index ec3fbae58d..5935b32fe3 100644
> >>>> --- a/include/hw/virtio/vhost-backend.h
> >>>> +++ b/include/hw/virtio/vhost-backend.h
> >>>> @@ -26,6 +26,18 @@ typedef enum VhostSetConfigType {
> >>>>        VHOST_SET_CONFIG_TYPE_MIGRATION = 1,
> >>>>    } VhostSetConfigType;
> >>>>
> >>>> +typedef enum VhostDeviceStateDirection {
> >>>> +    /* Transfer state from back-end (device) to front-end */
> >>>> +    VHOST_TRANSFER_STATE_DIRECTION_SAVE = 0,
> >>>> +    /* Transfer state from front-end to back-end (device) */
> >>>> +    VHOST_TRANSFER_STATE_DIRECTION_LOAD = 1,
> >>>> +} VhostDeviceStateDirection;
> >>>> +
> >>>> +typedef enum VhostDeviceStatePhase {
> >>>> +    /* The device (and all its vrings) is stopped */
> >>>> +    VHOST_TRANSFER_STATE_PHASE_STOPPED = 0,
> >>>> +} VhostDeviceStatePhase;
> >>> vDPA has:
> >>>
> >>>     /* Suspend a device so it does not process virtqueue requests anymore
> >>>      *
> >>>      * After the return of ioctl the device must preserve all the 
> >>> necessary state
> >>>      * (the virtqueue vring base plus the possible device specific 
> >>> states) that is
> >>>      * required for restoring in the future. The device must not change 
> >>> its
> >>>      * configuration after that point.
> >>>      */
> >>>     #define VHOST_VDPA_SUSPEND      _IO(VHOST_VIRTIO, 0x7D)
> >>>
> >>>     /* Resume a device so it can resume processing virtqueue requests
> >>>      *
> >>>      * After the return of this ioctl the device will have restored all 
> >>> the
> >>>      * necessary states and it is fully operational to continue 
> >>> processing the
> >>>      * virtqueue descriptors.
> >>>      */
> >>>     #define VHOST_VDPA_RESUME       _IO(VHOST_VIRTIO, 0x7E)
> >>>
> >>> I wonder if it makes sense to import these into vhost-user so that the
> >>> difference between kernel vhost and vhost-user is minimized. It's okay
> >>> if one of them is ahead of the other, but it would be nice to avoid
> >>> overlapping/duplicated functionality.
> >>>
> >>> (And I hope vDPA will import the device state vhost-user messages
> >>> introduced in this series.)
> >> I don’t understand your suggestion.  (Like, I very simply don’t
> >> understand :))
> >>
> >> These are vhost messages, right?  What purpose do you have in mind for
> >> them in vhost-user for internal migration?  They’re different from the
> >> state transfer messages, because they don’t transfer state to/from the
> >> front-end.  Also, the state transfer stuff is supposed to be distinct
> >> from starting/stopping the device; right now, it just requires the
> >> device to be stopped beforehand (or started only afterwards).  And in
> >> the future, new VhostDeviceStatePhase values may allow the messages to
> >> be used on devices that aren’t stopped.
> >>
> >> So they seem to serve very different purposes.  I can imagine using the
> >> VDPA_{SUSPEND,RESUME} messages for external migration (what Anton is
> >> working on), but they don’t really help with internal migration
> >> implemented here.  If I were to add them, they’d just be sent in
> >> addition to the new messages added in this patch here, i.e. SUSPEND on
> >> the source before SET_DEVICE_STATE_FD, and RESUME on the destination
> >> after CHECK_DEVICE_STATE (we could use RESUME in place of
> >> CHECK_DEVICE_STATE on the destination, but we can’t do that on the
> >> source, so we still need CHECK_DEVICE_STATE).
> > Yes, they are complementary to the device state fd message. I want to
> > make sure pre-conditions about the device's state (running vs stopped)
> > already take into account the vDPA SUSPEND/RESUME model.
> >
> > vDPA will need device state save/load in the future. For virtiofs
> > devices, for example. This is why I think we should plan for vDPA and
> > vhost-user to share the same interface.
>
> While the paragraph below is more important, I don’t feel like this
> would be important right now.  It’s clear that SUSPEND must come before
> transferring any state, and that RESUME must come after transferring
> state.  I don’t think we need to clarify this now, it’d be obvious when
> implementing SUSPEND/RESUME.
>
> > Also, I think the code path you're relying on (vhost_dev_stop()) on
> > doesn't work for backends that implement VHOST_USER_PROTOCOL_F_STATUS
> > because stopping the backend resets the device and throws away its
> > state. SUSPEND/RESUME solve this. This looks like a more general
> > problem since vhost_dev_stop() is called any time the VM is paused.
> > Maybe it needs to use SUSPEND/RESUME whenever possible.
>
> That’s a problem.  Quite a problem, to be honest, because this sounds
> rather complicated with honestly absolutely no practical benefit right
> now.
>
> Would you require SUSPEND/RESUME for state transfer even if the back-end
> does not implement GET/SET_STATUS?  Because then this would also lead to
> more complexity in virtiofsd.
>
> Basically, what I’m hearing is that I need to implement a different
> feature that has no practical impact right now, and also fix bugs around
> it along the way...

Eugenio's input regarding the design of the vhost-user messages is
important. That way we know it can be ported to vDPA later.

There is some extra discussion and work here, but only on the design
of the interface. You shouldn't need to implement extra unused stuff.
Whoever needs it can do that later based on a design that left room to
eventually do iterative migration for vhost-user and vDPA (comparable
to VFIO's migration interface).

Since both vDPA (vhost kernel) and vhost-user are stable APIs, it will
be hard to make significant design changes later without breaking all
existing implementations. That's why I think we should think ahead.

Stefan