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

["Li, Liang Z" <liang.z.li@intel.com>]

> >  }
> >
> > +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?

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

Will change.

Thanks!
Liang