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    SubjectRe: [PATCH 2/3] mlx5_vdpa: fix feature negotiation across device reset
    On Sat, Feb 06, 2021 at 04:29:23AM -0800, Si-Wei Liu wrote:
    > The mlx_features denotes the capability for which
    > set of virtio features is supported by device. In
    > principle, this field needs not be cleared during
    > virtio device reset, as this capability is static
    > and does not change across reset.
    >
    > In fact, the current code may have the assumption
    > that mlx_features can be reloaded from firmware
    > via the .get_features ops after device is reset
    > (via the .set_status ops), which is unfortunately
    > not true. The userspace VMM might save a copy
    > of backend capable features and won't call into
    > kernel again to get it on reset. This causes all
    > virtio features getting disabled on newly created
    > virtqs after device reset, while guest would hold
    > mismatched view of available features. For e.g.,
    > the guest may still assume tx checksum offload
    > is available after reset and feature negotiation,
    > causing frames with bogus (incomplete) checksum
    > transmitted on the wire.
    >
    > Signed-off-by: Si-Wei Liu <si-wei.liu@oracle.com>
    > ---
    > drivers/vdpa/mlx5/net/mlx5_vnet.c | 1 -
    > 1 file changed, 1 deletion(-)
    >
    > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
    > index b8416c4..aa6f8cd 100644
    > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
    > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
    > @@ -1788,7 +1788,6 @@ static void mlx5_vdpa_set_status(struct vdpa_device *vdev, u8 status)
    > clear_virtqueues(ndev);
    > mlx5_vdpa_destroy_mr(&ndev->mvdev);
    > ndev->mvdev.status = 0;
    > - ndev->mvdev.mlx_features = 0;
    > ++mvdev->generation;
    > return;
    > }

    Since we assume that device capabilities don't change, I think I would
    get the features through a call done in mlx5v_probe after the netdev
    object is created and change mlx5_vdpa_get_features() to just return
    ndev->mvdev.mlx_features.

    Did you actually see this issue in action? If you did, can you share
    with us how you trigerred this?

    > --
    > 1.8.3.1
    >

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