Re: [virtio-dev] Re: queue_enable vs QueueReady

[Jason Wang <jasowang@redhat.com>]

On 2020/6/2 下午12:20, Michael S. Tsirkin wrote:
> On Tue, Jun 02, 2020 at 10:57:19AM +0800, Jason Wang wrote:
>> On 2020/6/1 下午2:01, Michael S. Tsirkin wrote:
>>> On Thu, May 28, 2020 at 09:06:36PM +0800, Jason Wang wrote:
>>>> Hi:
>>>>
>>>> I found ambiguity in the virtio specification:
>>>>
>>>> In PCI part, it describes the queue_enable as:
>>>>
>>>> The driver uses this to selectively prevent the device from executing
>>>> requests from this virtqueue. 1 - enabled; 0 - disabled.
>>>>
>>>> In MMIO part, it describes the QueueReady as:
>>>>
>>>> Writing one (0x1) to this register notifies the device that it can execute
>>>> requests from this virtual queue. Reading from this register returns the
>>>> last value written to it. Both read and write accesses apply to the queue
>>>> selected by writing to QueueSel.
>>>>
>>>> If I understand this correctly, they have the same meaning, but the driver
>>>> requirements section looks conflict:
>>>>
>>>> PCI said: The driver MUST NOT write a 0 to queue_enable.
>>>>
>>>> MMIO said:
>>>>
>>>> To stop using the queue the driver MUST write zero (0x0) to this QueueReady
>>>> and MUST read the value back to ensure synchronization.
>>>>
>>>> So we can't disable a queue via queue_enable but QueueReady. Any reason for
>>>> such inconsistency?
>>>>
>>>> Thanks
>>> PCI assumed device reset is enough to stop all queues.  We had tons of
>>> bugs around shutdown because of this, so in hindsight, MMIO had maybe a
>>> better idea.
>>>
>>> Ability to stop a queue and take back buffers would be nice, e.g. serial
>>> is kind of messed up around port disconnect without it.
>>
>> Yes, but this inconsistency brings trouble for vDPA implementation which has
>> a set_queue_ready() and we do the following in virtio_vdpa as MMIO did:
>>
>>      /* Select and deactivate the queue */
>>      ops->set_vq_ready(vdpa, index, 0);
>>      WARN_ON(ops->get_vq_ready(vdpa, index));
>>
>> The codes seems to work fine for IFC (a violation of the spec probably) but
>> not Qemu's virtio-net-pci.
>>
>> Thanks
> What do you mean "not Qemu's virtio-net-pci"? Which implementation
> of .set_vq_ready do you use?


I meant the queue_enable implemented in qemu virtio-net-pci.

See my patch: [PATCH] virtio-pci: fix queue_enable write


>
> According to spec, I would expect this callback to do something
> other that writing into queue_enable.
>
> BTW I noticed this:
>
>          if (cmd == VHOST_VDPA_SET_VRING_ENABLE) {
>                  if (copy_from_user(&s, argp, sizeof(s)))
>                          return -EFAULT;
>                  ops->set_vq_ready(vdpa, idx, s.num);
>                  return 0;
>          }
>
> I'm guessing s.num should be 1, right?


It was a value set by userspace, so I think we can't have this assumption.

Thanks


>


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