Re: [PATCH v3 2/2] vdpa/mlx5: Make VIRTIO_NET_F_MRG_RXBUF off by default

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Mar 21, 2023 at 11:24:34AM +0200, Eli Cohen wrote:
> 
> On 20/03/2023 22:02, Michael S. Tsirkin wrote:
> > On Mon, Mar 20, 2023 at 01:49:30PM +0200, Eli Cohen wrote:
> > > One can still enable it when creating the vdpa device using vdpa tool by
> > > providing features that include it.
> > > 
> > > For example:
> > > $ vdpa dev add name vdpa0 mgmtdev pci/0000:86:00.2 device_features 0x300cb982b
> > > 
> > > Please be aware that this feature was not supported before the previous
> > > patch in this list was introduced so we don't change user experience.
> > so I would  say the patches have to be reordered to avoid a regression?
> Yes, I can do that.
> > 
> > > Current firmware versions show degradation in packet rate when using
> > > MRG_RXBUF. Users who favor memory saving over packet rate could enable
> > > this feature but we want to keep it off by default.
> > > 
> > > Signed-off-by: Eli Cohen <elic@nvidia.com>
> > OK and when future firmware will (maybe) fix this up how
> > will you know it's ok to enable by default?
> > Some version check I guess?
> > It would be better if firmware specified flags to enable
> > by default ...
> Currently there are no flags for that so let's just keep the default off.

Right but I mean, why are mrg buffers slow? It's a firmware bug right?

> > 
> > > ---
> > >   drivers/vdpa/mlx5/net/mlx5_vnet.c | 2 ++
> > >   1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > index 5285dd76c793..24397a71d6f3 100644
> > > --- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > +++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c
> > > @@ -3146,6 +3146,8 @@ static int mlx5_vdpa_dev_add(struct vdpa_mgmt_dev *v_mdev, const char *name,
> > >   			return -EINVAL;
> > >   		}
> > >   		device_features &= add_config->device_features;
> > > +	} else {
> > > +		device_features &= ~BIT_ULL(VIRTIO_NET_F_MRG_RXBUF);
> > >   	}
> > >   	if (!(device_features & BIT_ULL(VIRTIO_F_VERSION_1) &&
> > >   	      device_features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM))) {
> > > -- 
> > > 2.38.1

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