Re: [Qemu-devel] [PATCH v17 6/6] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ

[Wei Wang]

On 11/14/2017 01:32 AM, Michael S. Tsirkin wrote:
> You should Cc Nitesh who is working on a related feature.

OK, I'll do. We have two more issues which haven't been discussed yet, 
please have a check below.

> On Mon, Nov 13, 2017 at 06:34:48PM +0800, Wei Wang wrote:
> > Ping for comments, thanks.
> >
> > On 11/03/2017 04:13 PM, Wei Wang wrote:
> > > +static void virtballoon_cmd_report_free_page_start(struct virtio_balloon *vb)
> > > +{
> > > +       unsigned long flags;
> > > +
> > > +       vb->report_free_page_stop = false;
> this flag is used a lot outside any locks. Why is this safe?
> Please add some comments explaining access to this flag.

I will revert the logic as suggested: vb->report_free_page. Also plan to 
simplify its usage as below.

The flag is set or cleared in the config handler according to the 
new_cmd_id given
by the host:

new_cmd_id=0:                    WRITE_ONCE(vb->report_free_page, 
false); // stop reporting
new_cmd_id != old_cmd_id: WRITE_ONCE(vb->report_free_page, true);  // 
start reporting


The flag is read by virtio_balloon_send_free_pages() - the callback to 
report free pages:

if (!READ_ONCE(vb->report_free_page))
                return false;

I don't find where it could be unsafe then (the flag is written by the 
config handler only).



> > > +}
> > > +
> > >    static inline s64 towards_target(struct virtio_balloon *vb)
> > >    {
> > >         s64 target;
> > > @@ -597,42 +673,147 @@ static void update_balloon_size_func(struct work_struct 
> > > *work)
> > >                 queue_work(system_freezable_wq, work);
> > >    }
> > > -static int init_vqs(struct virtio_balloon *vb)
> > > +static bool virtio_balloon_send_free_pages(void *opaque, unsigned long pfn,
> > > +                                          unsigned long nr_pages)
> > >    {
> > > -       struct virtqueue *vqs[3];
> > > -       vq_callback_t *callbacks[] = { balloon_ack, balloon_ack, stats_request 
> > > };
> > > -       static const char * const names[] = { "inflate", "deflate", "stats" };
> > > -       int err, nvqs;
> > > +       struct virtio_balloon *vb = (struct virtio_balloon *)opaque;
> > > +       void *addr = (void *)pfn_to_kaddr(pfn);
> How do we know all free pages have a kaddr?

For x86_64, it works well since the kernel has all the physical memory 
mapped already. But for 32-bit kernel, yes, the high memory usually 
isn't mapped and thus no kaddr. Essentially, this pfn_to_kaddr convert 
isn't necessary, we do it here because the current API that virtio has 
is based on "struct scatterlist", which takes a kaddr, and this kaddr is 
then convert back to physical address in virtqueue_add() when assigning 
to desc->addr.

I think a better solution would be to add a new API, which directly 
assigns the caller's guest physical address to desc->addr, similar to 
the previous implementation "add_one_chunk()" 
(https://lists.gnu.org/archive/html/qemu-devel/2017-06/msg02452.html). 
But we can change that to a general virtio API:
virtqueue_add_one_desc(struct virtqueue *_vq, u64 base_addr, u32 size, 
bool in_desc, void *data);

What do you think?

Best,
Wei