Re: [RFC v4 3/5] virtio_ring: add packed ring support

[Jason Wang <jasowang@redhat.com>]

On 2018年05月16日 21:45, Tiwei Bie wrote:
> On Wed, May 16, 2018 at 08:51:43PM +0800, Jason Wang wrote:
>> On 2018年05月16日 20:39, Tiwei Bie wrote:
>>> On Wed, May 16, 2018 at 07:50:16PM +0800, Jason Wang wrote:
>>>> On 2018年05月16日 16:37, Tiwei Bie wrote:
>>> [...]
>>>>>     struct vring_virtqueue {
>>>>> @@ -116,6 +117,9 @@ struct vring_virtqueue {
>>>>>     			/* Last written value to driver->flags in
>>>>>     			 * guest byte order. */
>>>>>     			u16 event_flags_shadow;
>>>>> +
>>>>> +			/* ID allocation. */
>>>>> +			struct idr buffer_id;
>>>> I'm not sure idr is fit for the performance critical case here. Need to
>>>> measure its performance impact, especially if we have few unused slots.
>>> I'm also not sure.. But fortunately, it should be quite easy
>>> to replace it with something else without changing other code.
>>> If it will really hurt the performance, I'll change it.
>> We may want to do some benchmarking/profiling to see.
> Yeah!
>
>>>>>     		};
>>>>>     	};
>>> [...]
>>>>> +static void detach_buf_packed(struct vring_virtqueue *vq, unsigned int head,
>>>>> +			      unsigned int id, void **ctx)
>>>>> +{
>>>>> +	struct vring_packed_desc *desc;
>>>>> +	unsigned int i, j;
>>>>> +
>>>>> +	/* Clear data ptr. */
>>>>> +	vq->desc_state[id].data = NULL;
>>>>> +
>>>>> +	i = head;
>>>>> +
>>>>> +	for (j = 0; j < vq->desc_state[id].num; j++) {
>>>>> +		desc = &vq->vring_packed.desc[i];
>>>>> +		vring_unmap_one_packed(vq, desc);
>>>> As mentioned in previous discussion, this probably won't work for the case
>>>> of out of order completion since it depends on the information in the
>>>> descriptor ring. We probably need to extend ctx to record such information.
>>> Above code doesn't depend on the information in the descriptor
>>> ring. The vq->desc_state[] is the extended ctx.
>>>
>>> Best regards,
>>> Tiwei Bie
>> Yes, but desc is a pointer to descriptor ring I think so
>> vring_unmap_one_packed() still depends on the content of descriptor ring?
>>
> I got your point now. I think it makes sense to reserve
> the bits of the addr field. Driver shouldn't try to get
> addrs from the descriptors when cleanup the descriptors
> no matter whether we support out-of-order or not.

Maybe I was wrong, but I remember spec mentioned something like this.

>
> But combining it with the out-of-order support, it will
> mean that the driver still needs to maintain a desc/ctx
> list that is very similar to the desc ring in the split
> ring. I'm not quite sure whether it's something we want.
> If it is true, I'll do it. So do you think we also want
> to maintain such a desc/ctx list for packed ring?

To make it work for OOO backends I think we need something like this 
(hardware NIC drivers are usually have something like this).

Not for the patch, but it looks like having a OUT_OF_ORDER feature bit 
is much more simpler to be started with.

Thanks

>
> Best regards,
> Tiwei Bie