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

From: Jason Wang
Date: Mon Feb 22 2021 - 21:05:07 EST



On 2021/2/23 1:09 上午, Si-Wei Liu wrote:


On 2/21/2021 8:14 PM, Jason Wang wrote:

On 2021/2/19 7:54 下午, Si-Wei Liu wrote:
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.


This looks like a spec violation:

"

The following driver-read-only field, mtu only exists if VIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for the driver to use.
"

Do we really want to workaround this?

Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?


Yes, but the problem is we can't detect whether or not it's an legacy guest (e.g feature is not set).



I think the point is, since there's legacy guest we'd have to support, this host side workaround is unavoidable.


Since from vhost-vDPA point of view the driver is Qemu, it means we need make qemu vhost-vDPA driver spec complaint.

E.g how about:

1) revert 452639a64ad8 and fix Qemu? In Qemu, during vhost-vDPA initialization, do a minial config sync by neogitating minimal features (e.g just VIRTIO_F_ACCESS_PLATFORM). When FEATURE_OK is not set from guest, we can only allow it to access the config space that is emulated by Qemu?

Then


Although I agree the violating driver should be fixed (yes, it's in today's upstream kernel which exists for a while now).


2) Fix the virtio driver bug.

Or a quick workaround is to set (VIRTIO_F_ACCESS_PLATFORM instead of 0) in this case.

Thanks



-Siwei


Thanks


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 <si-wei.liu@xxxxxxxxxx>
---
  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;
-}
-
  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)