Re: [Qemu-devel] [PATCH v2 repost 7/7] virtio-balloon: tell host vm's free page info

[Michael S. Tsirkin]

On Thu, Jul 28, 2016 at 07:50:52AM +0000, Li, Liang Z wrote:
> > >  }
> > >
> > > +static void update_free_pages_stats(struct virtio_balloon *vb,
> > 
> > why _stats?
> 
> Will change.
> 
> > > + max_pfn = get_max_pfn();
> > > + mutex_lock(&vb->balloon_lock);
> > > + while (pfn < max_pfn) {
> > > +         memset(vb->page_bitmap, 0, vb->bmap_len);
> > > +         ret = get_free_pages(pfn, pfn + vb->pfn_limit,
> > > +                 vb->page_bitmap, vb->bmap_len * BITS_PER_BYTE);
> > > +         hdr->cmd = cpu_to_virtio16(vb->vdev,
> > BALLOON_GET_FREE_PAGES);
> > > +         hdr->page_shift = cpu_to_virtio16(vb->vdev, PAGE_SHIFT);
> > > +         hdr->req_id = cpu_to_virtio64(vb->vdev, req_id);
> > > +         hdr->start_pfn = cpu_to_virtio64(vb->vdev, pfn);
> > > +         bmap_len = vb->pfn_limit / BITS_PER_BYTE;
> > > +         if (!ret) {
> > > +                 hdr->flag = cpu_to_virtio16(vb->vdev,
> > > +
> >     BALLOON_FLAG_DONE);
> > > +                 if (pfn + vb->pfn_limit > max_pfn)
> > > +                         bmap_len = (max_pfn - pfn) /
> > BITS_PER_BYTE;
> > > +         } else
> > > +                 hdr->flag = cpu_to_virtio16(vb->vdev,
> > > +
> >     BALLOON_FLAG_CONT);
> > > +         hdr->bmap_len = cpu_to_virtio64(vb->vdev, bmap_len);
> > > +         sg_init_one(&sg_out, hdr,
> > > +                  sizeof(struct balloon_bmap_hdr) + bmap_len);
> > 
> > Wait a second. This adds the same buffer multiple times in a loop.
> > We will overwrite the buffer without waiting for hypervisor to process it.
> > What did I miss?
> 
> I am no quite sure about this part, I though the virtqueue_kick(vq) will 
> prevent
> the buffer from overwrite, I realized it's wrong.
> 
> > > +
> > > +         virtqueue_add_outbuf(vq, &sg_out, 1, vb, GFP_KERNEL);
> > 
> > this can fail. you want to maybe make sure vq has enough space before you
> > use it or check error and wait.
> > 
> > > +         virtqueue_kick(vq);
> > 
> > why kick here within loop? wait until done. in fact kick outside lock is 
> > better
> > for smp.
> 
> I will change this part in v3.
> 
> > 
> > > +         pfn += vb->pfn_limit;
> > > + static const char * const names[] = { "inflate", "deflate", "stats",
> > > +                                          "misc" };
> > >   int err, nvqs;
> > >
> > >   /*
> > >    * We expect two virtqueues: inflate and deflate, and
> > >    * optionally stat.
> > >    */
> > > - nvqs = virtio_has_feature(vb->vdev,
> > VIRTIO_BALLOON_F_STATS_VQ) ? 3 : 2;
> > > + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_MISC_VQ))
> > > +         nvqs = 4;
> > 
> > Does misc vq depend on stats vq feature then? if yes please validate that.
> 
> Yes, what's you mean by 'validate' that?

Either handle misc vq without a stats vq, or
clear VIRTIO_BALLOON_F_MISC_VQ if stats vq is off.

> > 
> > 
> > > + else
> > > +         nvqs = virtio_has_feature(vb->vdev,
> > > +                                   VIRTIO_BALLOON_F_STATS_VQ) ? 3 :
> > 2;
> > 
> > Replace that ? with else too pls.
> 
> Will change.
> 
> Thanks!
> Liang