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

[Jason Wang <jasowang@redhat.com>]

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?


> This
> implies that a given field, if it exists, is always at the same offset
> from the beginning of the configuration space."
>
>
>>> (Do we need to specify what a device needs to do if the driver tries to
>>> access a non-existing field? We cannot really make assumptions about
>>> config space accesses; virtio-ccw can just copy a chunk of config space
>>> that contains non-existing fields...
>>
>> Right, it looks to me ccw doesn't depends on config len which is kind of
>> interesting. I think the answer depends on the implementation of both
>> transport and device:
> (virtio-ccw is a bit odd, because channel I/O does not have the concept
> of a config space and I needed to come up with something)


Ok.


>
>> Let's take virtio-net-pci as an example, it fills status unconditionally
>> in virtio_net_set_config() even if VIRTIO_NET_F_STATUS is not
>> negotiated. So with the above feature_sizes:
>>
>> 1) if one of the MQ, MTU, DUPLEX or RSS is implemented, driver can still
>> read status in this case
>> 2) if none of the above four is negotied, driver can only read a 0xFF
>> (virtio_config_readb())
>>
>>
>>> I guess the device could ignore
>>> writes and return zeroes on read?)
>>
>> So consider the above, it might be too late to fix/clarify that in the
>> spec on the expected behaviour of reading/writing non-exist fields.
> We could still advise behaviour via SHOULD, though I'm not sure it adds
> much at this point in time.
>
> It really feels a bit odd that a driver can still read and write fields
> for features that it did not negotiate, but I fear we're stuck with it.


Yes, since the device (at least virtio-pci) implment thing like this.

Thanks


>
>> Thanks
>>
>>
>>> I've opened https://github.com/oasis-tcs/virtio-spec/issues/98 for the
>>> spec issues.
>>>   
>
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