Re: [Qemu-devel] [PATCH v7 4/5] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_HINT

[Wei Wang]

On 06/06/2018 02:43 PM, Peter Xu wrote:
> On Tue, Apr 24, 2018 at 02:13:47PM +0800, Wei Wang wrote:
>
> [...]
>
> +        if (elem->out_num) {
> +            size = iov_to_buf(elem->out_sg, elem->out_num, 0, &id, sizeof(id));
> +            virtqueue_push(vq, elem, size);
> Silly question: is this sending the same id back to guest?  Why?

No. It's just giving back the used buffer.

> > +            g_free(elem);
> > +
> > +            virtio_tswap32s(vdev, &id);
> > +            if (unlikely(size != sizeof(id))) {
> > +                virtio_error(vdev, "received an incorrect cmd id");
> Forgot to unlock?
>
> Maybe we can just move the lock operations outside:
>
>    mutex_lock();
>    while (1) {
>      ...
>      if (block) {
>        qemu_cond_wait();
>      }
>      ...
>      if (skip) {
>        continue;
>      }
>      ...
>      if (error) {
>        break;
>      }
>      ...
>    }
>    mutex_unlock();


I got similar comments from Michael, and it will be
while (1) {
lock;
func();
unlock();
}

All the unlock inside the body will be gone.

> [...]
> > +static const VMStateDescription vmstate_virtio_balloon_free_page_report = {
> > +    .name = "virtio-balloon-device/free-page-report",
> > +    .version_id = 1,
> > +    .minimum_version_id = 1,
> > +    .needed = virtio_balloon_free_page_support,
> > +    .fields = (VMStateField[]) {
> > +        VMSTATE_UINT32(free_page_report_cmd_id, VirtIOBalloon),
> > +        VMSTATE_UINT32(poison_val, VirtIOBalloon),
> (could we move all the poison-related lines into another patch or
>   postpone?  after all we don't support it yet, do we?)

 We don't support migrating poison value, but guest maybe use it, so we 
are actually disabling this feature in that case. Probably good to leave 
the code together to handle that case.


> > +    if (virtio_has_feature(s->host_features,
> > +                           VIRTIO_BALLOON_F_FREE_PAGE_HINT)) {
> > +        s->free_page_vq = virtio_add_queue(vdev, VIRTQUEUE_MAX_SIZE, NULL);
> > +        s->free_page_report_status = FREE_PAGE_REPORT_S_STOP;
> > +        s->free_page_report_cmd_id =
> > +                           VIRTIO_BALLOON_FREE_PAGE_REPORT_CMD_ID_MIN - 1;
> Why explicitly -1?  I thought ID_MIN would be fine too?

Yes, that will also be fine. Since we states that the cmd id will be 
from [MIN, MAX], and we make s->free_page_report_cmd_id++ in start(), 
using VIRTIO_BALLOON_FREE_PAGE_REPORT_CMD_ID_MIN here will make it [MIN 
+ 1, MAX].

> > +        if (s->iothread) {
> > +            object_ref(OBJECT(s->iothread));
> > +            s->free_page_bh = aio_bh_new(iothread_get_aio_context(s->iothread),
> > +                                       virtio_balloon_poll_free_page_hints, s);
> Just to mention that now we can create internal iothreads.  Please
> have a look at iothread_create().

Thanks. I noticed that, but I think configuring via the cmd line can let 
people share the iothread with other devices that need it.

Best,
Wei