Netdev Archive on lore.kernel.org
 help / color / Atom feed
From: Jason Wang <jasowang@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>,
	Si-Wei Liu <si-wei.liu@oracle.com>
Cc: elic@nvidia.com, linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org,
	netdev@vger.kernel.org, virtio-dev@lists.oasis-open.org
Subject: Re: [PATCH] vdpa/mlx5: set_features should allow reset to zero
Date: Tue, 23 Feb 2021 17:46:20 +0800
Message-ID: <788a0880-0a68-20b7-5bdf-f8150b08276a@redhat.com> (raw)
In-Reply-To: <20210223041740-mutt-send-email-mst@kernel.org>


On 2021/2/23 下午5:25, 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:
>>> 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?
>>
>> 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 a
>> while 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?


>          }
>
>          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 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”.
>
>
> 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 
the spec that has different implications.

Thanks


>
> Generally 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.
>


  reply index

Thread overview: 64+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-19 11:54 Si-Wei Liu
2021-02-21 14:44 ` Eli Cohen
2021-02-21 21:52   ` Michael S. Tsirkin
2021-02-22  6:05     ` Eli Cohen
2021-02-23  9:26       ` Michael S. Tsirkin
2021-02-23  9:48         ` Jason Wang
2021-02-23  9:55           ` Michael S. Tsirkin
2021-02-22  4:14 ` Jason Wang
2021-02-22  7:34   ` Michael S. Tsirkin
2021-02-23  1:12     ` Si-Wei Liu
2021-02-23  2:03       ` Jason Wang
2021-02-23 13:26         ` Michael S. Tsirkin
2021-02-23 19:35           ` Si-Wei Liu
2021-02-24  3:20             ` Jason Wang
2021-02-24  5:17               ` Michael S. Tsirkin
2021-02-24  6:02                 ` Jason Wang
2021-02-24  6:45                 ` Eli Cohen
2021-02-24  6:47                   ` Michael S. Tsirkin
2021-02-24  6:55                     ` Jason Wang
2021-02-24  7:12                       ` Michael S. Tsirkin
2021-02-24 12:40                         ` Eli Cohen
2021-02-24  7:17                       ` Eli Cohen
2021-02-24  5:04             ` Michael S. Tsirkin
2021-02-24  6:04               ` Jason Wang
2021-02-24  6:46                 ` Michael S. Tsirkin
2021-02-24  6:53                   ` Jason Wang
2021-02-24  7:17                     ` Michael S. Tsirkin
     [not found]                       ` <babc654d-8dcd-d8a2-c3b6-d20cc4fc554c@redhat.com>
2021-02-24  8:43                         ` Michael S. Tsirkin
2021-02-24  9:30                           ` Jason Wang
2021-02-28 21:30                             ` Michael S. Tsirkin
2021-03-01  3:53                               ` Jason Wang
2021-02-24 18:24               ` Si-Wei Liu
2021-02-26  0:56                 ` Si-Wei Liu
2021-02-28 21:27                   ` Michael S. Tsirkin
2021-03-01 18:08                     ` Si-Wei Liu
2021-02-28 21:28                 ` Michael S. Tsirkin
2021-02-28 21:34                 ` Michael S. Tsirkin
2021-03-01  3:56                   ` Jason Wang
2021-03-02  9:47                     ` Michael S. Tsirkin
2021-03-02 10:53                       ` Jason Wang
2021-02-22 17:09   ` Si-Wei Liu
2021-02-23  2:03     ` Jason Wang
2021-02-23  9:25     ` Michael S. Tsirkin
2021-02-23  9:46       ` Jason Wang [this message]
2021-02-23 10:01         ` Michael S. Tsirkin
2021-02-23 10:17           ` Jason Wang
2021-02-24  9:40             ` Jason Wang
2021-02-23 10:04         ` [virtio-dev] " Cornelia Huck
2021-02-23 10:31           ` Jason Wang
2021-02-23 10:58             ` Cornelia Huck
2021-02-24  9:29               ` Jason Wang
2021-02-24 11:12                 ` Cornelia Huck
2021-02-25  4:36                   ` Jason Wang
2021-02-25 13:26                     ` Cornelia Huck
2021-02-25 18:53                     ` Michael S. Tsirkin
2021-02-26  8:19                       ` Jason Wang
2021-02-28 21:25                         ` Michael S. Tsirkin
2021-03-01  3:51                           ` Jason Wang
2021-03-02 12:08                             ` Cornelia Huck
     [not found]                               ` <5f6972fe-7246-b622-958d-9cab8dd98e21@redhat.com>
2021-03-03  8:29                                 ` Cornelia Huck
2021-03-04  8:24                                   ` Jason Wang
2021-03-04 13:50                                     ` Cornelia Huck
2021-03-05  3:01                                       ` Jason Wang
2021-02-23 12:26 ` Michael S. Tsirkin

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=788a0880-0a68-20b7-5bdf-f8150b08276a@redhat.com \
    --to=jasowang@redhat.com \
    --cc=elic@nvidia.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=mst@redhat.com \
    --cc=netdev@vger.kernel.org \
    --cc=si-wei.liu@oracle.com \
    --cc=virtio-dev@lists.oasis-open.org \
    --cc=virtualization@lists.linux-foundation.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

Netdev Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/netdev/0 netdev/git/0.git
	git clone --mirror https://lore.kernel.org/netdev/1 netdev/git/1.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 netdev netdev/ https://lore.kernel.org/netdev \
		netdev@vger.kernel.org
	public-inbox-index netdev

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.netdev


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git