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

From: Jason Wang
Date: Fri May 18 2018 - 20:16:40 EST



On 2018å05æ18æ 22:33, Tiwei Bie wrote:
On Fri, May 18, 2018 at 09:17:05PM +0800, Jason Wang wrote:
On 2018å05æ18æ 19:29, Tiwei Bie wrote:
On Thu, May 17, 2018 at 08:01:52PM +0800, Jason Wang wrote:
On 2018å05æ16æ 22:33, Tiwei Bie wrote:
On Wed, May 16, 2018 at 10:05:44PM +0800, Jason Wang wrote:
On 2018å05æ16æ 21:45, Tiwei Bie wrote:
On Wed, May 16, 2018 at 08:51:43PM +0800, Jason Wang wrote:
On 2018å05æ16æ 20:39, Tiwei Bie wrote:
On Wed, May 16, 2018 at 07:50:16PM +0800, Jason Wang wrote:
On 2018å05æ16æ 16:37, Tiwei Bie wrote:
[...]
+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.
You're right. Spec mentioned this. I was just repeating
the spec to emphasize that it does make sense. :)

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).
Which hardware NIC drivers have this?
It's quite common I think, e.g driver track e.g dma addr and page frag
somewhere. e.g the ring->rx_info in mlx4 driver.
It seems that I had a misunderstanding on your
previous comments. I know it's quite common for
drivers to track e.g. DMA addrs somewhere (and
I think one reason behind this is that they want
to reuse the bits of addr field).
Yes, we may want this for virtio-net as well in the future.

But tracking
addrs somewhere doesn't means supporting OOO.
I thought you were saying it's quite common for
hardware NIC drivers to support OOO (i.e. NICs
will return the descriptors OOO):

I'm not familiar with mlx4, maybe I'm wrong.
I just had a quick glance. And I found below
comments in mlx4_en_process_rx_cq():

```
/* We assume a 1:1 mapping between CQEs and Rx descriptors, so Rx
* descriptor offset can be deduced from the CQE index instead of
* reading 'cqe->index' */
index = cq->mcq.cons_index & ring->size_mask;
cqe = mlx4_en_get_cqe(cq->buf, index, priv->cqe_size) + factor;
```

It seems that although they have a completion
queue, they are still using the ring in order.
I guess so (at least from the above bits). Git grep -i "out of order" in
drivers/net gives some hints. Looks like there're few deivces do this.

I guess maybe storage device may want OOO.
Right, some iSCSI did.

But tracking them elsewhere is not only for OOO.

Spec said:

for element address

"
In a used descriptor, Element Address is unused.
"

for Next flag:

"
For example, if descriptors are used in the same order in which they are
made available, this will result in
the used descriptor overwriting the first available descriptor in the list,
the used descriptor for the next list
overwriting the first available descriptor in the next list, etc.
"

for in order completion:

"
This will result in the used descriptor overwriting the first available
descriptor in the batch, the used descriptor
for the next batch overwriting the first available descriptor in the next
batch, etc.
"

So:

- It's an alignment to the spec
- device may (or should) overwrite the descriptor make also make address
field useless.
You didn't get my point...

I don't hope so.

I agreed driver should track the DMA addrs or some
other necessary things from the very beginning. And
I also repeated the spec to emphasize that it does
make sense. And I'd like to do that.

What I was saying is that, to support OOO, we may
need to manage these context (which saves DMA addrs
etc) via a list which is similar to the desc list
maintained via `next` in split ring instead of an
array whose elements always can be indexed directly.

My point is these context is a must (not only for OOO).


The desc ring in split ring is an array, but its
free entries are managed as list via next. I was
just wondering, do we want to manage such a list
because of OOO. It's just a very simple question
that I want to hear your opinion... (It doesn't
means anything, e.g. It doesn't mean I don't want
to support OOO. It's just a simple question...)

So the question is yes. But I admit I don't have better idea other than what you propose here (something like split ring which is a little bit sad). Maybe Michael had.

Thanks


Best regards,
Tiwei Bie

Thanks

Best regards,
Tiwei Bie

Thanks

Not for the patch, but it looks like having a OUT_OF_ORDER feature bit is
much more simpler to be started with.
+1

Best regards,
Tiwei Bie