Re: [PATCH 1/2] vdpa/mlx5: Fix suspend/resume index restoration

[Si-Wei Liu]

On 2/17/2021 11:42 AM, Si-Wei Liu wrote:
> On 2/16/2021 8:20 AM, Eli Cohen wrote:
> > When we suspend the VM, the VDPA interface will be reset. When the VM is
> > resumed again, clear_virtqueues() will clear the available and used
> > indices resulting in hardware virqtqueue objects becoming out of sync.
> > We can avoid this function alltogether since qemu will clear them if
> > required, e.g. when the VM went through a reboot.
> >
> > Moreover, since the hw available and used indices should always be
> > identical on query and should be restored to the same value same value
> > for virtqueues that complete in order, we set the single value provided
> > by set_vq_state(). In get_vq_state() we return the value of hardware
> > used index.
> >
> > Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 
> > devices")
> > Signed-off-by: Eli Cohen <elic@xxxxxxxxxx>
> Acked-by: Si-Wei Liu <si-wei.liu@xxxxxxxxxx>

I'd take it back for the moment, according to Jason's latest comment. 
Discussion still going.

> > ---
> >   drivers/vdpa/mlx5/net/mlx5_vnet.c | 17 ++++-------------
> >   1 file changed, 4 insertions(+), 13 deletions(-)
> >
> > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c 
> > b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > index b8e9d525d66c..a51b0f86afe2 100644
> > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > @@ -1169,6 +1169,7 @@ static void suspend_vq(struct mlx5_vdpa_net 
> > *ndev, struct mlx5_vdpa_virtqueue *m
> >           return;
> >       }
> >       mvq->avail_idx = attr.available_index;
> > +    mvq->used_idx = attr.used_index;
> >   }
> >     static void suspend_vqs(struct mlx5_vdpa_net *ndev)
> > @@ -1426,6 +1427,7 @@ static int mlx5_vdpa_set_vq_state(struct 
> > vdpa_device *vdev, u16 idx,
> >           return -EINVAL;
> >       }
> >   +    mvq->used_idx = state->avail_index;
> >       mvq->avail_idx = state->avail_index;
This is where things starts getting interesting. According to Jason, the 
original expectation of this API would be to restore the internal 
last_avail_index in the hardware. With Mellanox network vDPA hardware 
implementation, there's no such last_avail_index thing in the hardware 
but only a single last_used_index representing both indices, which 
should always be the same and in sync with each other. So from migration 
point of view, it appears logical to restore the saved last_avail_index 
to the last_used_index in the hardware, right? But what is the point to 
restore mvq->avail_idx?

Actually, this code path is being repurposed to address the index reset 
problem in the device reset scenario. Where the mvq->avail_idx and 
mvq->used_idx are both reset to 0 once device is reset. This is a bit 
crossing the boundary to me.


> >       return 0;
> >   }
> > @@ -1443,7 +1445,7 @@ static int mlx5_vdpa_get_vq_state(struct 
> > vdpa_device *vdev, u16 idx, struct vdpa
> >        * that cares about emulating the index after vq is stopped.
> >        */
> >       if (!mvq->initialized) {
> > -        state->avail_index = mvq->avail_idx;
> > +        state->avail_index = mvq->used_idx;
This is where the last_used_index gets loaded from the hardware (when 
device is stopped).

> >           return 0;
> >       }
> >   @@ -1452,7 +1454,7 @@ static int mlx5_vdpa_get_vq_state(struct 
> > vdpa_device *vdev, u16 idx, struct vdpa
> >           mlx5_vdpa_warn(mvdev, "failed to query virtqueue\n");
> >           return err;
> >       }
> > -    state->avail_index = attr.available_index;
> > +    state->avail_index = attr.used_index;
This code path never gets called from userspace (when device is running).

-Siwei

> >       return 0;
> >   }
> >   @@ -1532,16 +1534,6 @@ static void teardown_virtqueues(struct 
> > mlx5_vdpa_net *ndev)
> >       }
> >   }
> >   -static void clear_virtqueues(struct mlx5_vdpa_net *ndev)
> > -{
> > -    int i;
> > -
> > -    for (i = ndev->mvdev.max_vqs - 1; i >= 0; i--) {
> > -        ndev->vqs[i].avail_idx = 0;
> > -        ndev->vqs[i].used_idx = 0;
> > -    }
> > -}
> > -
> >   /* TODO: cross-endian support */
> >   static inline bool mlx5_vdpa_is_little_endian(struct mlx5_vdpa_dev 
> > *mvdev)
> >   {
> > @@ -1777,7 +1769,6 @@ static void mlx5_vdpa_set_status(struct 
> > vdpa_device *vdev, u8 status)
> >       if (!status) {
> >           mlx5_vdpa_info(mvdev, "performing device reset\n");
> >           teardown_driver(ndev);
> > -        clear_virtqueues(ndev);
> >           mlx5_vdpa_destroy_mr(&ndev->mvdev);
> >           ndev->mvdev.status = 0;
> >           ++mvdev->generation;