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

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

On Friday, November 26, 2021 12:11 AM, David Hildenbrand wrote:
> On 25.11.21 17:09, Michael S. Tsirkin wrote:
> > 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

Yes, it works based on the fact that Linux only sends one hint each time.

> >>
> >> 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.
> 
> Yeah, as I said, I don't particularly care. It's certainly an "easy change".
> 
> Acked-by: David Hildenbrand <david@redhat.com>
> 

Don’t object the change.
Just in case something unexpected, it would be better if someone could help do a test.

Thanks,
Wei