Re: [RFC v2] virtio: support packed ring

[Jason Wang <jasowang@redhat.com>]

On 2018年04月24日 09:05, Michael S. Tsirkin wrote:
>>>>> +	if (vq->indirect) {
>>>>> +		u32 len;
>>>>> +
>>>>> +		desc = vq->desc_state[head].indir_desc;
>>>>> +		/* Free the indirect table, if any, now that it's unmapped. */
>>>>> +		if (!desc)
>>>>> +			goto out;
>>>>> +
>>>>> +		len = virtio32_to_cpu(vq->vq.vdev,
>>>>> +				      vq->vring_packed.desc[head].len);
>>>>> +
>>>>> +		BUG_ON(!(vq->vring_packed.desc[head].flags &
>>>>> +			 cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_INDIRECT)));
>>>> It looks to me spec does not force to keep VRING_DESC_F_INDIRECT here. So we
>>>> can safely remove this BUG_ON() here.
>>>>
>>>>> +		BUG_ON(len == 0 || len % sizeof(struct vring_packed_desc));
>>>> Len could be ignored for used descriptor according to the spec, so we need
>>>> remove this BUG_ON() too.
>>> Yeah, you're right! The BUG_ON() isn't right. I'll remove it.
>>> And I think something related to this in the spec isn't very
>>> clear currently.
>>>
>>> In the spec, there are below words:
>>>
>>> https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L272
>>> """
>>> In descriptors with VIRTQ_DESC_F_INDIRECT set VIRTQ_DESC_F_WRITE
>>> is reserved and is ignored by the device.
>>> """
>>>
>>> So when device writes back an used descriptor in this case,
>>> device may not set the VIRTQ_DESC_F_WRITE flag as the flag
>>> is reserved and should be ignored.
>>>
>>> https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L170
>>> """
>>> Element Length is reserved for used descriptors without the
>>> VIRTQ_DESC_F_WRITE flag, and is ignored by drivers.
>>> """
>>>
>>> And this is the way how driver ignores the `len` in an used
>>> descriptor.
>>>
>>> https://github.com/oasis-tcs/virtio-spec/blob/d4fec517dfcf/packed-ring.tex#L241
>>> """
>>> To increase ring capacity the driver can store a (read-only
>>> by the device) table of indirect descriptors anywhere in memory,
>>> and insert a descriptor in the main virtqueue (with \field{Flags}
>>> bit VIRTQ_DESC_F_INDIRECT on) that refers to a buffer element
>>> containing this indirect descriptor table;
>>> """
>>>
>>> So the indirect descriptors in the table are read-only by
>>> the device. And the only descriptor which is writeable by
>>> the device is the descriptor in the main virtqueue (with
>>> Flags bit VIRTQ_DESC_F_INDIRECT on). So if we ignore the
>>> `len` in this descriptor, we won't be able to get the
>>> length of the data written by the device.
>>>
>>> So I think the `len` in this descriptor will carry the
>>> length of the data written by the device (if the buffers
>>> are writable to the device) even if the VIRTQ_DESC_F_WRITE
>>> isn't set by the device. How do you think?
>> Yes I think so. But we'd better need clarification from Michael.
> I think if you use a descriptor, and you want to supply len
> to guest, you set VIRTQ_DESC_F_WRITE in the used descriptor.
> Spec also says you must not set VIRTQ_DESC_F_INDIRECT then.
> If that's a problem we could look at relaxing that last requirement -
> does driver want INDIRECT in used descriptor to match
> the value in the avail descriptor for some reason?
>

Looks not, so what I get it:

- device and set VIRTQ_DESC_F_WRITE flag for used descriptor when needed
- there no need to keep INDIRECT flag in used descriptor

So for the above case, we can just have a used descriptor with _F_WRITE 
but without INDIRECT flag.

Thanks