Re: [RFC v2 11/13] vdpa: add vdpa net migration state notifier

[Si-Wei Liu <si-wei.liu@oracle.com>]

On 1/12/2023 9:24 AM, Eugenio Pérez wrote:
> This allows net to restart the device backend to configure SVQ on it.
>
> Ideally, these changes should not be net specific. However, the vdpa net
> backend is the one with enough knowledge to configure everything because
> of some reasons:
> * Queues might need to be shadowed or not depending on its kind (control
>    vs data).
> * Queues need to share the same map translations (iova tree).
>
> Because of that it is cleaner to restart the whole net backend and
> configure again as expected, similar to how vhost-kernel moves between
> userspace and passthrough.
>
> If more kinds of devices need dynamic switching to SVQ we can create a
> callback struct like VhostOps and move most of the code there.
> VhostOps cannot be reused since all vdpa backend share them, and to
> personalize just for networking would be too heavy.
>
> Signed-off-by: Eugenio Pérez <eperezma@redhat.com>
> ---
>   net/vhost-vdpa.c | 84 ++++++++++++++++++++++++++++++++++++++++++++++++
>   1 file changed, 84 insertions(+)
>
> diff --git a/net/vhost-vdpa.c b/net/vhost-vdpa.c
> index 5d7ad6e4d7..f38532b1df 100644
> --- a/net/vhost-vdpa.c
> +++ b/net/vhost-vdpa.c
> @@ -26,6 +26,8 @@
>   #include <err.h>
>   #include "standard-headers/linux/virtio_net.h"
>   #include "monitor/monitor.h"
> +#include "migration/migration.h"
> +#include "migration/misc.h"
>   #include "migration/blocker.h"
>   #include "hw/virtio/vhost.h"
>   
> @@ -33,6 +35,7 @@
>   typedef struct VhostVDPAState {
>       NetClientState nc;
>       struct vhost_vdpa vhost_vdpa;
> +    Notifier migration_state;
>       Error *migration_blocker;
>       VHostNetState *vhost_net;
>   
> @@ -243,10 +246,86 @@ static VhostVDPAState *vhost_vdpa_net_first_nc_vdpa(VhostVDPAState *s)
>       return DO_UPCAST(VhostVDPAState, nc, nc0);
>   }
>   
> +static void vhost_vdpa_net_log_global_enable(VhostVDPAState *s, bool enable)
> +{
> +    struct vhost_vdpa *v = &s->vhost_vdpa;
> +    VirtIONet *n;
> +    VirtIODevice *vdev;
> +    int data_queue_pairs, cvq, r;
> +    NetClientState *peer;
> +
> +    /* We are only called on the first data vqs and only if x-svq is not set */
> +    if (s->vhost_vdpa.shadow_vqs_enabled == enable) {
> +        return;
> +    }
> +
> +    vdev = v->dev->vdev;
> +    n = VIRTIO_NET(vdev);
> +    if (!n->vhost_started) {
> +        return;
> +    }
> +
> +    if (enable) {
> +        ioctl(v->device_fd, VHOST_VDPA_SUSPEND);
> +    }
> +    data_queue_pairs = n->multiqueue ? n->max_queue_pairs : 1;
> +    cvq = virtio_vdev_has_feature(vdev, VIRTIO_NET_F_CTRL_VQ) ?
> +                                  n->max_ncs - n->max_queue_pairs : 0;
> +    vhost_net_stop(vdev, n->nic->ncs, data_queue_pairs, cvq);
> +
> +    peer = s->nc.peer;
> +    for (int i = 0; i < data_queue_pairs + cvq; i++) {
> +        VhostVDPAState *vdpa_state;
> +        NetClientState *nc;
> +
> +        if (i < data_queue_pairs) {
> +            nc = qemu_get_peer(peer, i);
> +        } else {
> +            nc = qemu_get_peer(peer, n->max_queue_pairs);
> +        }
> +
> +        vdpa_state = DO_UPCAST(VhostVDPAState, nc, nc);
> +        vdpa_state->vhost_vdpa.shadow_data = enable;
> +
> +        if (i < data_queue_pairs) {
> +            /* Do not override CVQ shadow_vqs_enabled */
> +            vdpa_state->vhost_vdpa.shadow_vqs_enabled = enable;
> +        }
> +    }
> +
> +    r = vhost_net_start(vdev, n->nic->ncs, data_queue_pairs, cvq);
As the first revision, this method (vhost_net_stop followed by 
vhost_net_start) should be fine for software vhost-vdpa backend for e.g. 
vp_vdpa and vdpa_sim_net. However, I would like to get your attention 
that this method implies substantial blackout time for mode switching on 
real hardware - get a full cycle of device reset of getting memory 
mappings torn down, unpin & repin same set of pages, and set up new 
mapping would take very significant amount of time, especially for a 
large VM. Maybe we can do:

1) replace reset with the RESUME feature that was just added to the 
vhost-vdpa ioctls in kernel
2) add new vdpa ioctls to allow iova range rebound to new virtual 
address for QEMU's shadow vq or back to device's vq
3) use a light-weighted sequence of suspend+rebind+resume to switch mode 
on the fly instead of getting through the whole reset+restart cycle

I suspect the same idea could even be used to address high live 
migration downtime seen on hardware vdpa device. What do you think?

Thanks,
-Siwei

> +    if (unlikely(r < 0)) {
> +        error_report("unable to start vhost net: %s(%d)", g_strerror(-r), -r);
> +    }
> +}
> +
> +static void vdpa_net_migration_state_notifier(Notifier *notifier, void *data)
> +{
> +    MigrationState *migration = data;
> +    VhostVDPAState *s = container_of(notifier, VhostVDPAState,
> +                                     migration_state);
> +
> +    switch (migration->state) {
> +    case MIGRATION_STATUS_SETUP:
> +        vhost_vdpa_net_log_global_enable(s, true);
> +        return;
> +
> +    case MIGRATION_STATUS_CANCELLING:
> +    case MIGRATION_STATUS_CANCELLED:
> +    case MIGRATION_STATUS_FAILED:
> +        vhost_vdpa_net_log_global_enable(s, false);
> +        return;
> +    };
> +}
> +
>   static void vhost_vdpa_net_data_start_first(VhostVDPAState *s)
>   {
>       struct vhost_vdpa *v = &s->vhost_vdpa;
>   
> +    if (v->feature_log) {
> +        add_migration_state_change_notifier(&s->migration_state);
> +    }
> +
>       if (v->shadow_vqs_enabled) {
>           v->iova_tree = vhost_iova_tree_new(v->iova_range.first,
>                                              v->iova_range.last);
> @@ -280,6 +359,10 @@ static void vhost_vdpa_net_client_stop(NetClientState *nc)
>   
>       assert(nc->info->type == NET_CLIENT_DRIVER_VHOST_VDPA);
>   
> +    if (s->vhost_vdpa.index == 0 && s->vhost_vdpa.feature_log) {
> +        remove_migration_state_change_notifier(&s->migration_state);
> +    }
> +
>       dev = s->vhost_vdpa.dev;
>       if (dev->vq_index + dev->nvqs == dev->vq_index_end) {
>           g_clear_pointer(&s->vhost_vdpa.iova_tree, vhost_iova_tree_delete);
> @@ -767,6 +850,7 @@ static NetClientState *net_vhost_vdpa_init(NetClientState *peer,
>       s->vhost_vdpa.device_fd = vdpa_device_fd;
>       s->vhost_vdpa.index = queue_pair_index;
>       s->always_svq = svq;
> +    s->migration_state.notify = vdpa_net_migration_state_notifier;
>       s->vhost_vdpa.shadow_vqs_enabled = svq;
>       s->vhost_vdpa.iova_range = iova_range;
>       s->vhost_vdpa.shadow_data = svq;