[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]
Subject: Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
On 2021/2/23 6:17 äå, Jason Wang wrote:
On 2021/2/23 6:01 äå, Michael S. Tsirkin wrote:On Tue, Feb 23, 2021 at 05:46:20PM +0800, Jason Wang wrote:On 2021/2/23 äå5:25, Michael S. Tsirkin wrote:On Mon, Feb 22, 2021 at 09:09:28AM -0800, Si-Wei Liu wrote:On 2/21/2021 8:14 PM, Jason Wang wrote:Isn't the commit 452639a64ad8 itself is a workaround for legacy guest?On 2021/2/19 7:54 äå, 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 ifVIRTIO_NET_F_MTU is set. This field specifies the maximum MTU for thedriver to use. " Do we really want to workaround this?I think the point is, since there's legacy guest we'd have to support, this host side workaround is unavoidable. Although I agree the violating driver should be fixed (yes, it's in today's upstream kernel which exists for awhile now).Oh you are right: static int virtnet_validate(struct virtio_device *vdev) {  if (!vdev->config->get) { dev_err(&vdev->dev, "%s failure: config access disabled\n", __func__);  return -EINVAL;  }  if (!virtnet_validate_features(vdev))  return -EINVAL;  if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) {  int mtu = virtio_cread16(vdev, offsetof(struct virtio_net_config, mtu));  if (mtu < MIN_MTU)  __virtio_clear_bit(vdev, VIRTIO_NET_F_MTU);I wonder why not simply fail here?Back in 2016 it went like this: ÂÂÂÂOn Thu, Jun 02, 2016 at 05:10:59PM -0400, Aaron Conole wrote: ÂÂÂÂ> + if (virtio_has_feature(vdev, VIRTIO_NET_F_MTU)) { ÂÂÂÂ> + dev->mtu = virtio_cread16(vdev,ÂÂÂÂ> + offsetof(struct virtio_net_config,ÂÂÂÂ> + mtu)); ÂÂÂÂ> + } ÂÂÂÂ> + ÂÂÂÂ> if (vi->any_header_sg) ÂÂÂÂ> dev->needed_headroom = vi->hdr_len; ÂÂÂÂ> ÂÂÂÂOne comment though: I think we should validate the mtu. ÂÂÂÂIf it's invalid, clear VIRTIO_NET_F_MTU and ignore. Too late at this point :) I guess it's a way to tell device "I can not live with this MTU", device can fail FEATURES_OK if it wants to. MIN_MTU is an internal linux thing and at the time I felt it's better to try to make progress.What if e.g the device advertise a large MTU. E.g 64K here?
Ok, consider we use add_recvbuf_small() when neither GSO nor mrg_rxbuf. This means we should fail the probing if MTU is greater than 1500 in this case.
Thanks
In that case, the driver can not live either. Clearing MTU won't help here.ThanksÂÂÂÂÂÂÂÂÂ } ÂÂÂÂÂÂÂÂÂ return 0; } And the spec says: 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 configurationfields 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 notsupport 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 deviceâs virtio configuration space, and population of virtqueues.8. Set the DRIVER_OK status bit. At this point the device is âliveâ. Item 4 on the list explicitly allows reading config space before FEATURES_OK.I conclude that VIRTIO_NET_F_MTU is set means "set in device features".So this probably need some clarification. "is set" is used many times in thespec that has different implications. ThanksGenerally it is worth going over feature dependent config fields and checking whether they should be present when device feature is set or when feature bit has been negotiated, and making this clear.
[Date Prev] | [Thread Prev] | [Thread Next] | [Date Next] -- [Date Index] | [Thread Index] | [List Home]