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Subject: Re: [virtio-dev] Re: [PATCH] vdpa/mlx5: set_features should allow
 reset to zero
To:     Cornelia Huck <cohuck@redhat.com>
Cc:     "Michael S. Tsirkin" <mst@redhat.com>,
        Si-Wei Liu <si-wei.liu@oracle.com>, elic@nvidia.com,
        linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        virtio-dev@lists.oasis-open.org
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From:   Jason Wang <jasowang@redhat.com>
Message-ID: <71fd3f74-00ba-f085-27e9-6a0d21c9a93f@redhat.com>
Date:   Fri, 5 Mar 2021 11:01:28 +0800
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On 2021/3/4 9:50 下午, Cornelia Huck wrote:
> On Thu, 4 Mar 2021 16:24:16 +0800
> Jason Wang <jasowang@redhat.com> wrote:
>
>> On 2021/3/3 4:29 下午, Cornelia Huck wrote:
>>> On Wed, 3 Mar 2021 12:01:01 +0800
>>> Jason Wang <jasowang@redhat.com> wrote:
>>>   
>>>> On 2021/3/2 8:08 下午, Cornelia Huck wrote:
>>>>> On Mon, 1 Mar 2021 11:51:08 +0800
>>>>> Jason Wang <jasowang@redhat.com> wrote:
>>>>>      
>>>>>> On 2021/3/1 5:25 上午, Michael S. Tsirkin wrote:
>>>>>>> On Fri, Feb 26, 2021 at 04:19:16PM +0800, Jason Wang wrote:
>>>>>>>> On 2021/2/26 2:53 上午, Michael S. Tsirkin wrote:
>>>>>>>>> Confused. What is wrong with the above? It never reads the
>>>>>>>>> field unless the feature has been offered by device.
>>>>>>>> So the spec said:
>>>>>>>>
>>>>>>>> "
>>>>>>>>
>>>>>>>> The following driver-read-only field, max_virtqueue_pairs only exists if
>>>>>>>> VIRTIO_NET_F_MQ is set.
>>>>>>>>
>>>>>>>> "
>>>>>>>>
>>>>>>>> If I read this correctly, there will be no max_virtqueue_pairs field if the
>>>>>>>> VIRTIO_NET_F_MQ is not offered by device? If yes the offsetof() violates
>>>>>>>> what spec said.
>>>>>>>>
>>>>>>>> Thanks
>>>>>>> I think that's a misunderstanding. This text was never intended to
>>>>>>> imply that field offsets change beased on feature bits.
>>>>>>> We had this pain with legacy and we never wanted to go back there.
>>>>>>>
>>>>>>> This merely implies that without VIRTIO_NET_F_MQ the field
>>>>>>> should not be accessed. Exists in the sense "is accessible to driver".
>>>>>>>
>>>>>>> Let's just clarify that in the spec, job done.
>>>>>> Ok, agree. That will make things more eaiser.
>>>>> Yes, that makes much more sense.
>>>>>
>>>>> What about adding the following to the "Basic Facilities of a Virtio
>>>>> Device/Device Configuration Space" section of the spec:
>>>>>
>>>>> "If an optional configuration field does not exist, the corresponding
>>>>> space is still present, but reserved."
>>>> This became interesting after re-reading some of the qemu codes.
>>>>
>>>> E.g in virtio-net.c we had:
>>>>
>>>> *static VirtIOFeature feature_sizes[] = {
>>>>        {.flags = 1ULL << VIRTIO_NET_F_MAC,
>>>>         .end = endof(struct virtio_net_config, mac)},
>>>>        {.flags = 1ULL << VIRTIO_NET_F_STATUS,
>>>>         .end = endof(struct virtio_net_config, status)},
>>>>        {.flags = 1ULL << VIRTIO_NET_F_MQ,
>>>>         .end = endof(struct virtio_net_config, max_virtqueue_pairs)},
>>>>        {.flags = 1ULL << VIRTIO_NET_F_MTU,
>>>>         .end = endof(struct virtio_net_config, mtu)},
>>>>        {.flags = 1ULL << VIRTIO_NET_F_SPEED_DUPLEX,
>>>>         .end = endof(struct virtio_net_config, duplex)},
>>>>        {.flags = (1ULL << VIRTIO_NET_F_RSS) | (1ULL <<
>>>> VIRTIO_NET_F_HASH_REPORT),
>>>>         .end = endof(struct virtio_net_config, supported_hash_types)},
>>>>        {}
>>>> };*
>>>>
>>>> *It has a implict dependency chain. E.g MTU doesn't presnet if
>>>> DUPLEX/RSS is not offered ...
>>>> *
>>> But I think it covers everything up to the relevant field, no? So MTU
>>> is included if we have the feature bit, even if we don't have
>>> DUPLEX/RSS.
>>>
>>> Given that a config space may be shorter (but must not collapse
>>> non-existing fields), maybe a better wording would be:
>>>
>>> "If an optional configuration field does not exist, the corresponding
>>> space will still be present if it is not at the end of the
>>> configuration space (i.e., further configuration fields exist.)
>>
>> This should work but I think we need to define the end of configuration
>> space first?
> What about sidestepping this:
>
> "...the corresponding space will still be present, unless no further
> configuration fields exist."
>
> ?


It might work. (I wonder maybe we can give some example in the spec).

Thanks


>
>>> This
>>> implies that a given field, if it exists, is always at the same offset
>>> from the beginning of the configuration space."