Messages in this thread | | | Subject | Re: [RFC v4 3/5] virtio_ring: add packed ring support | From | Jason Wang <> | Date | Sat, 19 May 2018 09:12:30 +0800 |
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On 2018年05月18日 22:33, Tiwei Bie wrote: > On Fri, May 18, 2018 at 09:17:05PM +0800, Jason Wang wrote: >> On 2018年05月18日 19:29, Tiwei Bie wrote: >>> On Thu, May 17, 2018 at 08:01:52PM +0800, Jason Wang wrote: >>>> On 2018年05月16日 22:33, Tiwei Bie wrote: >>>>> On Wed, May 16, 2018 at 10:05:44PM +0800, Jason Wang wrote: >>>>>> 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: >>>>> [...] >>>>>>>>>>> +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
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