On Mon, Apr 17, 2023 at 5:38 PM Stefan Hajnoczi <stefa...@redhat.com> wrote: > > On Thu, Apr 13, 2023 at 12:14:24PM +0200, Eugenio Perez Martin wrote: > > On Wed, Apr 12, 2023 at 11:06 PM Stefan Hajnoczi <stefa...@redhat.com> > > 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 <stefa...@redhat.com> > > > > Signed-off-by: Hanna Czenczek <hre...@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. > > > > > > > That's what I had in mind in the first versions. I proposed VHOST_STOP > > instead of VHOST_VDPA_STOP for this very reason. Later it did change > > to SUSPEND. > > I noticed QEMU only calls ioctl(VHOST_VDPA_SUSPEND) and not > ioctl(VHOST_VDPA_RESUME). > > The doc comments in <linux/vdpa.h> don't explain how the device can > leave the suspended state. Can you clarify this? >
Do you mean in what situations or regarding the semantics of _RESUME? To me resume is an operation mainly to resume the device in the event of a VM suspension, not a migration. It can be used as a fallback code in some cases of migration failure though, but it is not currently used in qemu. Thanks!