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Tsirkin" To: Jason Wang Cc: Si-Wei Liu , Eugenio =?iso-8859-1?Q?P=E9rez?= , linux-kernel@vger.kernel.org, Dragos Tatulea , virtualization@lists.linux-foundation.org, leiyang@redhat.com, Xuan Zhuo Subject: Re: [PATCH] mlx5_vdpa: offer VHOST_BACKEND_F_ENABLE_AFTER_DRIVER_OK Message-ID: <20230705012917-mutt-send-email-mst@kernel.org> References: <20230703142514.363256-1-eperezma@redhat.com> <20230703110241-mutt-send-email-mst@kernel.org> <20230704061356-mutt-send-email-mst@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: X-Spam-Status: No, score=-2.1 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,RCVD_IN_DNSWL_NONE, RCVD_IN_MSPIKE_H4,RCVD_IN_MSPIKE_WL,SPF_HELO_NONE,SPF_NONE, T_SCC_BODY_TEXT_LINE autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on lindbergh.monkeyblade.net Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jul 05, 2023 at 01:11:37PM +0800, Jason Wang wrote: > On Tue, Jul 4, 2023 at 6:16 PM Michael S. Tsirkin 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 > > > > 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? But then at driver ok time we don't know how many queues are there. > > 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 It's just the 1st kick, then you can disable. No? > > > > > > > > > > > --- > > > > > 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 > > > > > #include > > > > > #include > > > > > +#include > > > > > #include > > > > > #include > > > > > #include > > > > > @@ -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 > >