Re: [PATCH 1/2] virito-balloon: process all in sgs for free_page_vq

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

On Thu, Nov 25, 2021 at 09:28:59AM +0100, David Hildenbrand wrote:
> On 25.11.21 03:20, Jason Wang wrote:
> > We only process the first in sg which may lead to the bitmap of the
> > pages belongs to following sgs were not cleared. This may result more
> > pages to be migrated. Fixing this by process all in sgs for
> > free_page_vq.
> > 
> > Signed-off-by: Jason Wang <jasowang@redhat.com>
> > ---
> >  hw/virtio/virtio-balloon.c | 7 +++++--
> >  1 file changed, 5 insertions(+), 2 deletions(-)
> > 
> > diff --git a/hw/virtio/virtio-balloon.c b/hw/virtio/virtio-balloon.c
> > index c6962fcbfe..17de2558cb 100644
> > --- a/hw/virtio/virtio-balloon.c
> > +++ b/hw/virtio/virtio-balloon.c
> > @@ -510,6 +510,7 @@ static bool get_free_page_hints(VirtIOBalloon *dev)
> >      VirtIODevice *vdev = VIRTIO_DEVICE(dev);
> >      VirtQueue *vq = dev->free_page_vq;
> >      bool ret = true;
> > +    int i;
> >  
> >      while (dev->block_iothread) {
> >          qemu_cond_wait(&dev->free_page_cond, &dev->free_page_lock);
> > @@ -544,8 +545,10 @@ static bool get_free_page_hints(VirtIOBalloon *dev)
> >      }
> >  
> >      if (elem->in_num && dev->free_page_hint_status == FREE_PAGE_HINT_S_START) {
> > -        qemu_guest_free_page_hint(elem->in_sg[0].iov_base,
> > -                                  elem->in_sg[0].iov_len);
> > +        for (i = 0; i < elem->in_num; i++) {
> > +            qemu_guest_free_page_hint(elem->in_sg[i].iov_base,
> > +                                      elem->in_sg[i].iov_len);
> > +        }
> >      }
> >  
> >  out:
> > 
> 
> Yes, but:
> 
> 1. Linux never used more than one
> 2. QEMU never consumed more than one
> 
> The spec states:
> 
> "(b) The driver maps a series of pages and adds them to the free_page_vq
> as individual scatter-gather input buffer entries."
> 
> However, the spec was written by someone else (Alex) as the code was
> (Wei). The code was there first.
> 
> I don't particularly care what to adjust (code or spec). However, to me
> it feels more like the spec is slightly wrong and it was intended like
> the code is by the original code author.
> 
> But then again, I don't particularly care :)

Original QEMU side code had several bugs so, that's another one.
Given nothing too bad happens if guest submits too many S/Gs,
and given the spec also has a general chapter suggesting devices
are flexible in accepting a single buffer split to multiple S/Gs,
I'm inclined to accept the patch.

> -- 
> Thanks,
> 
> David / dhildenb