Re: [PATCH v12 5/8] virtio-balloon: VIRTIO_BALLOON_F_SG

[Wei Wang <wei.w.wang@intel.com>]

On 07/23/2017 09:45 AM, Michael S. Tsirkin wrote:
> On Fri, Jul 14, 2017 at 03:12:43PM +0800, Wei Wang wrote:
>> On 07/14/2017 04:19 AM, Michael S. Tsirkin wrote:
>>> On Thu, Jul 13, 2017 at 03:42:35PM +0800, Wei Wang wrote:
>>>> On 07/12/2017 09:56 PM, Michael S. Tsirkin wrote:
>>>>> So the way I see it, there are several issues:
>>>>>
>>>>> - internal wait - forces multiple APIs like kick/kick_sync
>>>>>      note how kick_sync can fail but your code never checks return code
>>>>> - need to re-write the last descriptor - might not work
>>>>>      for alternative layouts which always expose descriptors
>>>>>      immediately
>>>> Probably it wasn't clear. Please let me explain the two functions here:
>>>>
>>>> 1) virtqueue_add_chain_desc(vq, head_id, prev_id,..):
>>>> grabs a desc from the vq and inserts it to the chain tail (which is indexed
>>>> by
>>>> prev_id, probably better to call it tail_id). Then, the new added desc
>>>> becomes
>>>> the tail (i.e. the last desc). The _F_NEXT flag is cleared for each desc
>>>> when it's
>>>> added to the chain, and set when another desc comes to follow later.
>>> And this only works if there are multiple rings like
>>> avail + descriptor ring.
>>> It won't work e.g. with the proposed new layout where
>>> writing out a descriptor exposes it immediately.
>> I think it can support the 1.1 proposal, too. But before getting
>> into that, I think we first need to deep dive into the implementation
>> and usage of _first/next/last. The usage would need to lock the vq
>> from the first to the end (otherwise, the returned info about the number
>> of available desc in the vq, i.e. num_free, would be invalid):
>>
>> lock(vq);
>> add_first();
>> add_next();
>> add_last();
>> unlock(vq);
>>
>> However, I think the case isn't this simple, since we need to check more
>> things
>> after each add_xx() step. For example, if only one entry is available at the
>> time
>> we start to use the vq, that is, num_free is 0 after add_first(), we
>> wouldn't be
>> able to add_next and add_last. So, it would work like this:
>>
>> start:
>>      ...get free page block..
>>      lock(vq)
>> retry:
>>      ret = add_first(..,&num_free,);
>>      if(ret == -ENOSPC) {
>>          goto retry;
>>      } else if (!num_free) {
>>          add_chain_head();
>>          unlock(vq);
>>          kick & wait;
>>          goto start;
>>      }
>> next_one:
>>      ...get free page block..
>>      add_next(..,&num_free,);
>>      if (!num_free) {
>>          add_chain_head();
>>          unlock(vq);
>>          kick & wait;
>>          goto start;
>>      } if (num_free == 1) {
>>          ...get free page block..
>>          add_last(..);
>>          unlock(vq);
>>          kick & wait;
>>          goto start;
>>      } else {
>>          goto next_one;
>>      }
>>
>> The above seems unnecessary to me to have three different APIs.
>> That's the reason to combine them into one virtqueue_add_chain_desc().
>>
>> -- or, do you have a different thought about using the three APIs?
>>
>>
>> Implementation Reference:
>>
>> struct desc_iterator {
>>      unsigned int head;
>>      unsigned int tail;
>> };
>>
>> add_first(*vq, *desc_iterator, *num_free, ..)
>> {
>>      if (vq->vq.num_free < 1)
>>          return -ENOSPC;
>>      get_desc(&desc_id);
>>      desc[desc_id].flag &= ~_F_NEXT;
>>      desc_iterator->head = desc_id
>>      desc_iterator->tail = desc_iterator->head;
>>      *num_free = vq->vq.num_free;
>> }
>>
>> add_next(vq, desc_iterator, *num_free,..)
>> {
>>      get_desc(&desc_id);
>>      desc[desc_id].flag &= ~_F_NEXT;
>>      desc[desc_iterator.tail].next = desc_id;
>>      desc[desc_iterator->tail].flag |= _F_NEXT;
>>      desc_iterator->tail = desc_id;
>>      *num_free = vq->vq.num_free;
>> }
>>
>> add_last(vq, desc_iterator,..)
>> {
>>      get_desc(&desc_id);
>>      desc[desc_id].flag &= ~_F_NEXT;
>>      desc[desc_iterator.tail].next = desc_id;
>>      desc_iterator->tail = desc_id;
>>
>>      add_chain_head(); // put the desc_iterator.head to the ring
>> }
>>
>>
>> Best,
>> Wei
> OK I thought this over. While we might need these new APIs in
> the future, I think that at the moment, there's a way to implement
> this feature that is significantly simpler. Just add each s/g
> as a separate input buffer.


Should it be an output buffer? I think output means from the
driver to device (i.e. DMA_TO_DEVICE).

>
> This needs zero new APIs.
>
> I know that follow-up patches need to add a header in front
> so you might be thinking: how am I going to add this
> header? The answer is quite simple - add it as a separate
> out header.
>
> Host will be able to distinguish between header and pages
> by looking at the direction, and - should we want to add
> IN data to header - additionally size (<4K => header).


I think this works fine when the cmdq is only used for
reporting the unused pages. It would be an issue
if there are other usages (e.g. report memory statistics)
interleaving. I think one solution would be to lock the cmdq until
a cmd usage is done ((e.g. all the unused pages are reported) ) -
in this case, the periodically updated guest memory statistics
may be delayed for a while occasionally when live migration starts.
Would this be acceptable? If not, probably we can have the cmdq
for one usage only.


Best,
Wei

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