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

[Dragos Tatulea <dtatulea@nvidia.com>]

On Fri, 2023-12-15 at 18:56 +0100, Eugenio Perez Martin wrote:
> On Fri, Dec 15, 2023 at 3:13 PM Dragos Tatulea <dtatulea@nvidia.com> wrote:
> > 
> > On Fri, 2023-12-15 at 12:35 +0000, Dragos Tatulea wrote:
> > > On Thu, 2023-12-14 at 19:30 +0100, Eugenio Perez Martin wrote:
> > > > On Thu, Dec 14, 2023 at 4:51 PM Dragos Tatulea <dtatulea@nvidia.com> wrote:
> > > > > 
> > > > > On Thu, 2023-12-14 at 08:45 -0500, Michael S. Tsirkin wrote:
> > > > > > On Thu, Dec 14, 2023 at 01:39:55PM +0000, Dragos Tatulea wrote:
> > > > > > > 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
> > > > > > 
> > > > > > I am fine with either of these.
> > > > > > 
> > > > > How about allowing the change only under the following conditions:
> > > > >   vhost_vdpa_can_suspend && vhost_vdpa_can_resume &&
> > > > > VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK is set
> > > > > 
> > > > > ?
> > > > 
> > > > I think the best option by far is 1, as there is no hint in the
> > > > combination of these 3 indicating that you can change device
> > > > properties in the suspended state.
> > > > 
> > > Sure. Will respin a v3 without these two patches.
> > > 
> > > Another series can implement option 2 and add these 2 patches on top.
> > Hmm...I misunderstood your statement and sent a erroneus v3. You said that
> > having a feature flag is the best option.
> > 
> > Will add a feature flag in v4: is this similar to the
> > VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK flag?
> > 
> 
> Right, it should be easy to return it from .get_backend_features op if
> the FW returns that capability, isn't it?
> 
Yes, that's easy. But I wonder if we need one feature bit for each type of 
change:
- VHOST_BACKEND_F_CHANGEABLE_VQ_ADDR_IN_SUSPEND
- VHOST_BACKEND_F_CHANGEABLE_VQ_STATE_IN_SUSPEND

Or would a big one VHOST_BACKEND_F_CAN_RECONFIG_VQ_IN_SUSPEND suffice?

To me having individual feature bits makes sense. But it could also takes too
many bits if more changes are required.

Thanks,
Dragos

> > Thanks,
> > Dragos
> > 
> > > > > Thanks,
> > > > > Dragos
> > > > > 
> > > > > > > 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
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