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

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 24 Feb 2021 02:17:59 -0500

On Wed, Feb 24, 2021 at 02:53:08PM +0800, Jason Wang wrote:
> 
> On 2021/2/24 2:46 下午, Michael S. Tsirkin wrote:
> > On Wed, Feb 24, 2021 at 02:04:36PM +0800, Jason Wang wrote:
> > > On 2021/2/24 1:04 下午, Michael S. Tsirkin wrote:
> > > > On Tue, Feb 23, 2021 at 11:35:57AM -0800, Si-Wei Liu wrote:
> > > > > On 2/23/2021 5:26 AM, Michael S. Tsirkin wrote:
> > > > > > On Tue, Feb 23, 2021 at 10:03:57AM +0800, Jason Wang wrote:
> > > > > > > On 2021/2/23 9:12 上午, Si-Wei Liu wrote:
> > > > > > > > On 2/21/2021 11:34 PM, Michael S. Tsirkin wrote:
> > > > > > > > > On Mon, Feb 22, 2021 at 12:14:17PM +0800, 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?
> > > > > > > > > > 
> > > > > > > > > > Thanks
> > > > > > > > > And also:
> > > > > > > > > 
> > > > > > > > > The driver MUST follow this sequence to initialize a device:
> > > > > > > > > 1. Reset the device.
> > > > > > > > > 2. Set the ACKNOWLEDGE status bit: the guest OS has noticed the device.
> > > > > > > > > 3. Set the DRIVER status bit: the guest OS knows how to drive the
> > > > > > > > > device.
> > > > > > > > > 4. Read device feature bits, and write the subset of feature bits
> > > > > > > > > understood by the OS and driver to the
> > > > > > > > > device. During this step the driver MAY read (but MUST NOT write)
> > > > > > > > > the device-specific configuration
> > > > > > > > > fields to check that it can support the device before accepting it.
> > > > > > > > > 5. Set the FEATURES_OK status bit. The driver MUST NOT accept new
> > > > > > > > > feature bits after this step.
> > > > > > > > > 6. Re-read device status to ensure the FEATURES_OK bit is still set:
> > > > > > > > > otherwise, the device does not
> > > > > > > > > support our subset of features and the device is unusable.
> > > > > > > > > 7. Perform device-specific setup, including discovery of virtqueues
> > > > > > > > > for the device, optional per-bus setup,
> > > > > > > > > reading and possibly writing the device’s virtio configuration
> > > > > > > > > space, and population of virtqueues.
> > > > > > > > > 8. Set the DRIVER_OK status bit. At this point the device is “live”.
> > > > > > > > > 
> > > > > > > > > 
> > > > > > > > > so accessing config space before FEATURES_OK is a spec violation, right?
> > > > > > > > It is, but it's not relevant to what this commit tries to address. I
> > > > > > > > thought the legacy guest still needs to be supported.
> > > > > > > > 
> > > > > > > > Having said, a separate patch has to be posted to fix the guest driver
> > > > > > > > issue where this discrepancy is introduced to virtnet_validate() (since
> > > > > > > > commit fe36cbe067). But it's not technically related to this patch.
> > > > > > > > 
> > > > > > > > -Siwei
> > > > > > > I think it's a bug to read config space in validate, we should move it to
> > > > > > > virtnet_probe().
> > > > > > > 
> > > > > > > Thanks
> > > > > > I take it back, reading but not writing seems to be explicitly allowed by spec.
> > > > > > So our way to detect a legacy guest is bogus, need to think what is
> > > > > > the best way to handle this.
> > > > > Then maybe revert commit fe36cbe067 and friends, and have QEMU detect legacy
> > > > > guest? Supposedly only config space write access needs to be guarded before
> > > > > setting FEATURES_OK.
> > > > > 
> > > > > -Siwie
> > > > Detecting it isn't enough though, we will need a new ioctl to notify
> > > > the kernel that it's a legacy guest. Ugh :(
> > > 
> > > I'm not sure I get this, how can we know if there's a legacy driver before
> > > set_features()?
> > qemu knows for sure. It does not communicate this information to the
> > kernel right now unfortunately.
> 
> 
> I may miss something, but I still don't get how the new ioctl is supposed to
> work.
> 
> Thanks

Basically on first guest access QEMU would tell kernel whether
guest is using the legacy or the modern interface.
E.g. virtio_pci_config_read/virtio_pci_config_write will call ioctl(ENABLE_LEGACY, 1)
while virtio_pci_common_read will call ioctl(ENABLE_LEGACY, 0)

Or maybe we just add GET_CONFIG_MODERN and GET_CONFIG_LEGACY and
call the correct ioctl ... there are many ways to build this API.

> 
> > 
> > > And I wonder what will hapeen if we just revert the set_features(0)?
> > > 
> > > 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 <[email-masked]>
> > > > > > > > > > > ---
> > > > > > > > > > >       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)