Re: [PATCH vhost v2 4/8] vdpa/mlx5: Mark vq addrs for modification in hw vq

[Dragos Tatulea <dtatulea@nvidia.com>]

On Tue, 2023-12-12 at 15:44 -0800, Si-Wei Liu wrote:
> 
> On 12/12/2023 11:21 AM, Eugenio Perez Martin wrote:
> > On Tue, Dec 5, 2023 at 11:46 AM Dragos Tatulea <dtatulea@nvidia.com> wrote:
> > > Addresses get set by .set_vq_address. hw vq addresses will be updated on
> > > next modify_virtqueue.
> > > 
> > > Signed-off-by: Dragos Tatulea <dtatulea@nvidia.com>
> > > Reviewed-by: Gal Pressman <gal@nvidia.com>
> > > Acked-by: Eugenio Pérez <eperezma@redhat.com>
> > I'm kind of ok with this patch and the next one about state, but I
> > didn't ack them in the previous series.
> > 
> > My main concern is that it is not valid to change the vq address after
> > DRIVER_OK in VirtIO, which vDPA follows. Only memory maps are ok to
> > change at this moment. I'm not sure about vq state in vDPA, but vhost
> > forbids changing it with an active backend.
> > 
> > Suspend is not defined in VirtIO at this moment though, so maybe it is
> > ok to decide that all of these parameters may change during suspend.
> > Maybe the best thing is to protect this with a vDPA feature flag.
> I think protect with vDPA feature flag could work, while on the other 
> hand vDPA means vendor specific optimization is possible around suspend 
> and resume (in case it helps performance), which doesn't have to be 
> backed by virtio spec. Same applies to vhost user backend features, 
> variations there were not backed by spec either. Of course, we should 
> try best to make the default behavior backward compatible with 
> virtio-based backend, but that circles back to no suspend definition in 
> the current virtio spec, for which I hope we don't cease development on 
> vDPA indefinitely. After all, the virtio based vdap backend can well 
> define its own feature flag to describe (minor difference in) the 
> suspend behavior based on the later spec once it is formed in future.
> 
So what is the way forward here? From what I understand the options are:

1) Add a vdpa feature flag for changing device properties while suspended.

2) Drop these 2 patches from the series for now. Not sure if this makes sense as
this. But then Si-Wei's qemu device suspend/resume poc [0] that exercises this
code won't work anymore. This means the series would be less well tested.

Are there other possible options? What do you think?

[0] https://github.com/siwliu-kernel/qemu/tree/svq-resume-wip

Thanks,
Dragos

> Regards,
> -Siwei
> 
> 
> 
> > 
> > Jason, what do you think?
> > 
> > Thanks!
> > 
> > > ---
> > >   drivers/vdpa/mlx5/net/mlx5_vnet.c  | 9 +++++++++
> > >   include/linux/mlx5/mlx5_ifc_vdpa.h | 1 +
> > >   2 files changed, 10 insertions(+)
> > > 
> > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > index f8f088cced50..80e066de0866 100644
> > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > @@ -1209,6 +1209,7 @@ static int modify_virtqueue(struct mlx5_vdpa_net *ndev,
> > >          bool state_change = false;
> > >          void *obj_context;
> > >          void *cmd_hdr;
> > > +       void *vq_ctx;
> > >          void *in;
> > >          int err;
> > > 
> > > @@ -1230,6 +1231,7 @@ static int modify_virtqueue(struct mlx5_vdpa_net *ndev,
> > >          MLX5_SET(general_obj_in_cmd_hdr, cmd_hdr, uid, ndev->mvdev.res.uid);
> > > 
> > >          obj_context = MLX5_ADDR_OF(modify_virtio_net_q_in, in, obj_context);
> > > +       vq_ctx = MLX5_ADDR_OF(virtio_net_q_object, obj_context, virtio_q_context);
> > > 
> > >          if (mvq->modified_fields & MLX5_VIRTQ_MODIFY_MASK_STATE) {
> > >                  if (!is_valid_state_change(mvq->fw_state, state, is_resumable(ndev))) {
> > > @@ -1241,6 +1243,12 @@ static int modify_virtqueue(struct mlx5_vdpa_net *ndev,
> > >                  state_change = true;
> > >          }
> > > 
> > > +       if (mvq->modified_fields & MLX5_VIRTQ_MODIFY_MASK_VIRTIO_Q_ADDRS) {
> > > +               MLX5_SET64(virtio_q, vq_ctx, desc_addr, mvq->desc_addr);
> > > +               MLX5_SET64(virtio_q, vq_ctx, used_addr, mvq->device_addr);
> > > +               MLX5_SET64(virtio_q, vq_ctx, available_addr, mvq->driver_addr);
> > > +       }
> > > +
> > >          MLX5_SET64(virtio_net_q_object, obj_context, modify_field_select, mvq->modified_fields);
> > >          err = mlx5_cmd_exec(ndev->mvdev.mdev, in, inlen, out, sizeof(out));
> > >          if (err)
> > > @@ -2202,6 +2210,7 @@ static int mlx5_vdpa_set_vq_address(struct vdpa_device *vdev, u16 idx, u64 desc_
> > >          mvq->desc_addr = desc_area;
> > >          mvq->device_addr = device_area;
> > >          mvq->driver_addr = driver_area;
> > > +       mvq->modified_fields |= MLX5_VIRTQ_MODIFY_MASK_VIRTIO_Q_ADDRS;
> > >          return 0;
> > >   }
> > > 
> > > diff --git a/include/linux/mlx5/mlx5_ifc_vdpa.h b/include/linux/mlx5/mlx5_ifc_vdpa.h
> > > index b86d51a855f6..9594ac405740 100644
> > > --- a/include/linux/mlx5/mlx5_ifc_vdpa.h
> > > +++ b/include/linux/mlx5/mlx5_ifc_vdpa.h
> > > @@ -145,6 +145,7 @@ enum {
> > >          MLX5_VIRTQ_MODIFY_MASK_STATE                    = (u64)1 << 0,
> > >          MLX5_VIRTQ_MODIFY_MASK_DIRTY_BITMAP_PARAMS      = (u64)1 << 3,
> > >          MLX5_VIRTQ_MODIFY_MASK_DIRTY_BITMAP_DUMP_ENABLE = (u64)1 << 4,
> > > +       MLX5_VIRTQ_MODIFY_MASK_VIRTIO_Q_ADDRS           = (u64)1 << 6,
> > >          MLX5_VIRTQ_MODIFY_MASK_DESC_GROUP_MKEY          = (u64)1 << 14,
> > >   };
> > > 
> > > --
> > > 2.42.0
> > > 
>