Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero

On Sun, Feb 21, 2021 at 04:44:37PM +0200, Eli Cohen wrote:

On Fri, Feb 19, 2021 at 06:54:58AM -0500, Si-Wei Liu wrote:

quoted

Commit 452639a64ad8 ("vdpa: make sure set_features is invoked
for legacy") made an exception for legacy guests to reset
features to 0, when config space is accessed before features
are set. We should relieve the verify_min_features() check
and allow features reset to 0 for this case.

It's worth noting that not just legacy guests could access
config space before features are set. For instance, when
feature VIRTIO_NET_F_MTU is advertised some modern driver
will try to access and validate the MTU present in the config
space before virtio features are set. Rejecting reset to 0
prematurely causes correct MTU and link status unable to load
for the very first config space access, rendering issues like
guest showing inaccurate MTU value, or failure to reject
out-of-range MTU.

Fixes: 1a86b377aa21 ("vdpa/mlx5: Add VDPA driver for supported mlx5 devices")
Signed-off-by: Si-Wei Liu <redacted>
---
 drivers/vdpa/mlx5/net/mlx5_vnet.c | 15 +--------------
 1 file changed, 1 insertion(+), 14 deletions(-)

diff --git a/drivers/vdpa/mlx5/net/mlx5_vnet.c b/drivers/vdpa/mlx5/net/mlx5_vnet.c
index 7c1f789..540dd67 100644
--- a/drivers/vdpa/mlx5/net/mlx5_vnet.c
+++ b/drivers/vdpa/mlx5/net/mlx5_vnet.c

@@ -1490,14 +1490,6 @@ static u64 mlx5_vdpa_get_features(struct vdpa_device *vdev)
 	return mvdev->mlx_features;
 }
 
-static int verify_min_features(struct mlx5_vdpa_dev *mvdev, u64 features)
-{
-	if (!(features & BIT_ULL(VIRTIO_F_ACCESS_PLATFORM)))
-		return -EOPNOTSUPP;
-
-	return 0;
-}
-

But what if VIRTIO_F_ACCESS_PLATFORM is not offerred? This does not
support such cases.

Did you mean "catch such cases" rather than "support"?

Maybe we should call verify_min_features() from mlx5_vdpa_set_status()
just before attempting to call setup_driver().

quoted

 static int setup_virtqueues(struct mlx5_vdpa_net *ndev)
 {
 	int err;

@@ -1558,18 +1550,13 @@ static int mlx5_vdpa_set_features(struct vdpa_device *vdev, u64 features)
 {
 	struct mlx5_vdpa_dev *mvdev = to_mvdev(vdev);
 	struct mlx5_vdpa_net *ndev = to_mlx5_vdpa_ndev(mvdev);
-	int err;
 
 	print_features(mvdev, features, true);
 
-	err = verify_min_features(mvdev, features);
-	if (err)
-		return err;
-
 	ndev->mvdev.actual_features = features & ndev->mvdev.mlx_features;
 	ndev->config.mtu = cpu_to_mlx5vdpa16(mvdev, ndev->mtu);
 	ndev->config.status |= cpu_to_mlx5vdpa16(mvdev, VIRTIO_NET_S_LINK_UP);
-	return err;
+	return 0;
 }
 
 static void mlx5_vdpa_set_config_cb(struct vdpa_device *vdev, struct vdpa_callback *cb)

-- 
1.8.3.1