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

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Jul 26, 2017 at 11:48:41AM +0800, Wei Wang wrote:
> 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?

Hypervisor overwrites these pages with zeroes. Therefore it is
writeable by device: DMA_FROM_DEVICE.

> I think output means from the
> driver to device (i.e. DMA_TO_DEVICE).

This part is correct I believe.

> > 
> > 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

OK I see, I think the issue is that reporting free pages
was structured like stats. Let's split it -
send pages on e.g. free_vq, get commands on vq shared with
stats.


-- 
MST