linux-kernel - Re: [PATCH] mlx5_vdpa: offer VHOST_BACKEND_F_ENABLE_AFTER_DRIVER

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Date:   Wed, 5 Jul 2023 13:11:37 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Si-Wei Liu <si-wei.liu@...cle.com>,
        Eugenio Pérez <eperezma@...hat.com>,
        linux-kernel@...r.kernel.org, Dragos Tatulea <dtatulea@...dia.com>,
        virtualization@...ts.linux-foundation.org, leiyang@...hat.com,
        Xuan Zhuo <xuanzhuo@...ux.alibaba.com>
Subject: Re: [PATCH] mlx5_vdpa: offer VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK

On Tue, Jul 4, 2023 at 6:16 PM Michael S. Tsirkin <mst@...hat.com> wrote:
>
> On Mon, Jul 03, 2023 at 05:26:02PM -0700, Si-Wei Liu wrote:
> >
> >
> > On 7/3/2023 8:46 AM, Michael S. Tsirkin wrote:
> > > On Mon, Jul 03, 2023 at 04:25:14PM +0200, Eugenio Pérez wrote:
> > > > Offer this backend feature as mlx5 is compatible with it. It allows it
> > > > to do live migration with CVQ, dynamically switching between passthrough
> > > > and shadow virtqueue.
> > > >
> > > > Signed-off-by: Eugenio Pérez <eperezma@...hat.com>
> > > Same comment.
> > to which?
> >
> > -Siwei
>
> VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK is too narrow a use-case to commit to it
> as a kernel/userspace ABI: what if one wants to start rings in some
> other specific order?

Just enable a queue by writing e.g 1 to queue_enable in a specific order?

> As was discussed on list, a better promise is not to access ring
> until the 1st kick. vdpa can then do a kick when it wants
> the device to start accessing rings.

Rethink about the ACCESS_AFTER_KICK, it sounds functional equivalent
to allow queue_enable after DRIVER_OK, but it seems to have
distanvages:

A busy polling software device may disable notifications and never
respond to register to any kick notifiers. ACCESS_AFTER_KICK will
introduce overhead to those implementations.

Thanks

>
> > >
> > > > ---
> > > >   drivers/vdpa/mlx5/net/mlx5_vnet.c | 7 +++++++
> > > >   1 file changed, 7 insertions(+)
> > > >
> > > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > index 9138ef2fb2c8..5f309a16b9dc 100644
> > > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > > @@ -7,6 +7,7 @@
> > > >   #include <uapi/linux/virtio_net.h>
> > > >   #include <uapi/linux/virtio_ids.h>
> > > >   #include <uapi/linux/vdpa.h>
> > > > +#include <uapi/linux/vhost_types.h>
> > > >   #include <linux/virtio_config.h>
> > > >   #include <linux/auxiliary_bus.h>
> > > >   #include <linux/mlx5/cq.h>
> > > > @@ -2499,6 +2500,11 @@ static void unregister_link_notifier(struct mlx5_vdpa_net *ndev)
> > > >                   flush_workqueue(ndev->mvdev.wq);
> > > >   }
> > > > +static u64 mlx5_vdpa_get_backend_features(const struct vdpa_device *vdpa)
> > > > +{
> > > > + return BIT_ULL(VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK);
> > > > +}
> > > > +
> > > >   static int mlx5_vdpa_set_driver_features(struct vdpa_device *vdev, u64 features)
> > > >   {
> > > >           struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
> > > > @@ -3140,6 +3146,7 @@ static const struct vdpa_config_ops mlx5_vdpa_ops = {
> > > >           .get_vq_align = mlx5_vdpa_get_vq_align,
> > > >           .get_vq_group = mlx5_vdpa_get_vq_group,
> > > >           .get_device_features = mlx5_vdpa_get_device_features,
> > > > + .get_backend_features = mlx5_vdpa_get_backend_features,
> > > >           .set_driver_features = mlx5_vdpa_set_driver_features,
> > > >           .get_driver_features = mlx5_vdpa_get_driver_features,
> > > >           .set_config_cb = mlx5_vdpa_set_config_cb,
> > > > --
> > > > 2.39.3
>