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Subject: Re: [PATCH v22 2/3] virtio-balloon: VIRTIO_BALLOON_F_FREE_PAGE_VQ
On Fri, Jan 19, 2018 at 11:44:21AM +0800, Wei Wang wrote:
> On 01/18/2018 12:44 AM, Michael S. Tsirkin wrote:
> > On Wed, Jan 17, 2018 at 01:10:11PM +0800, Wei Wang wrote:
> >
>
> >
> > > +{
> > > + struct scatterlist sg;
> > > + unsigned int unused;
> > > + int err;
> > > +
> > > + sg_init_one(&sg, addr, sizeof(uint32_t));
> > This passes a guest-endian value to host. This is a problem:
> > should always pass LE values.
>
> I think the endianness is handled when virtqueue_add_outbuf():
>
> desc[i].addr = cpu_to_virtio64(_vq->vdev, addr);
>
> right?
No - that handles the address, not the value you pass in.
> >
> > > +
> > > + /*
> > > + * This handles the cornercase that the vq happens to be full when
> > > + * adding a cmd id. Rarely happen in practice.
> > > + */
> > > + while (!vq->num_free)
> > > + virtqueue_get_buf(vq, &unused);
> > I dislike this busy-waiting. It's a hint after all -
> > why not just retry later - hopefully after getting an
> > interrupt?
> >
> > Alternatively, stop adding more entries when we have a single
> > ring entry left, making sure we have space for the command.
>
> I think the second one looks good. Thanks.
>
> > > + queue_work(system_freezable_wq,
> > > + &vb->update_balloon_size_work);
> > > + spin_unlock_irqrestore(&vb->stop_update_lock, flags);
> > > + }
> > > +
> > > + virtio_cread(vb->vdev, struct virtio_balloon_config,
> > > + free_page_report_cmd_id, &cmd_id);
> > You want virtio_cread_feature, don't access the new field
> > if the feature has not been negotiated.
>
> Right. We probably need to put all the following cmd id related things under
> the feature check,
>
> How about
>
> if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_VQ)) {
> virtio_cread(..);
> if (cmd_id == VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID) {
> ....
> }
>
that's ok too.
> >
> >
> > > + if (cmd_id == VIRTIO_BALLOON_FREE_PAGE_REPORT_STOP_ID) {
> > > + WRITE_ONCE(vb->report_free_page, false);
> > > + } else if (cmd_id != vb->start_cmd_id) {
> > > + /*
> > > + * Host requests to start the reporting by sending a new cmd
> > > + * id.
> > > + */
> > > + WRITE_ONCE(vb->report_free_page, true);
> > I don't know why we bother with WRITE_ONCE here. The point of
> > report_free_page being used lockless is that that it's not a big deal if
> > it's wrong occasionally, right?
>
> Actually the main reason is that "vb->report_free_page" is a value shared by
> two threads:
> Written by the config_change here, and read by the worker thread that
> reports the free pages.
Right but what's wrong if it's read or written twice and not once?
> Alternatively, we could let the two sides access to the shared variable with
> "volatile" pointers.
>
>
> >
> >
> >
> > > + vb->start_cmd_id = cmd_id;
> > > + queue_work(vb->balloon_wq, &vb->report_free_page_work);
> > It seems that if a command was already queued (with a different id),
> > this will result in new command id being sent to host twice, which will
> > likely confuse the host.
>
> I think that case won't happen, because
> - the host sends a cmd id to the guest via the config, while the guest acks
> back the received cmd id via the virtqueue;
> - the guest ack back a cmd id only when a new cmd id is received from the
> host, that is the above check:
>
> if (cmd_id != vb->start_cmd_id) { --> the driver only queues the
> reporting work only when a new cmd id is received
> /*
> * Host requests to start the reporting by sending a
> * new cmd id.
> */
> WRITE_ONCE(vb->report_free_page, true);
> vb->start_cmd_id = cmd_id;
> queue_work(vb->balloon_wq,
> &vb->report_free_page_work);
> }
>
> So the same cmd id wouldn't queue the reporting work twice.
>
Like this:
vb->start_cmd_id = cmd_id;
queue_work(vb->balloon_wq, &vb->report_free_page_work);
command id changes
vb->start_cmd_id = cmd_id;
work executes
queue_work(vb->balloon_wq, &vb->report_free_page_work);
work executes again
> >
> >
> >
> > > + }
> > > +}
> > > +
> > > static void update_balloon_size(struct virtio_balloon *vb)
> > > {
> > > u32 actual = vb->num_pages;
> > > @@ -417,40 +513,113 @@ static void update_balloon_size_func(struct work_struct *work)
> > > static int init_vqs(struct virtio_balloon *vb)
> > > {
> > > - 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 virtqueue **vqs;
> > > + vq_callback_t **callbacks;
> > > + const char **names;
> > > + struct scatterlist sg;
> > > + int i, nvqs, err = -ENOMEM;
> > > +
> > > + /* Inflateq and deflateq are used unconditionally */
> > > + nvqs = 2;
> > > + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ))
> > > + nvqs++;
> > > + if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_VQ))
> > > + nvqs++;
> > > +
> > > + /* Allocate space for find_vqs parameters */
> > > + vqs = kcalloc(nvqs, sizeof(*vqs), GFP_KERNEL);
> > > + if (!vqs)
> > > + goto err_vq;
> > > + callbacks = kmalloc_array(nvqs, sizeof(*callbacks), GFP_KERNEL);
> > > + if (!callbacks)
> > > + goto err_callback;
> > > + names = kmalloc_array(nvqs, sizeof(*names), GFP_KERNEL);
> > > + if (!names)
> > > + goto err_names;
> > Why not just keep these 3 arrays on stack? they aren't large.
>
> Sounds good. Here is the new implementation:
>
> static int init_vqs(struct virtio_balloon *vb)
> {
> struct virtqueue *vqs[4];
> vq_callback_t *callbacks[4];
> const char *names[4];
> struct scatterlist sg;
> int ret;
>
>
> /*
> * Inflateq and deflateq are used unconditionally. stats_vq and
> * free_page_vq uses names[2] and names[3], respectively. The
> names[]
> * will be NULL if the related feature is not enabled, which will
> * cause no allocation for the corresponding virtqueue in find_vqs.
> */
> callbacks[0] = balloon_ack;
> names[0] = "inflate";
> callbacks[1] = balloon_ack;
> names[1] = "deflate";
> names[2] = NULL;
> names[3] = NULL;
>
> if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
> names[2] = "stats";
> callbacks[2] = stats_request;
> }
> if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_VQ)) {
> names[3] = "free_page_vq";
> callbacks[3] = NULL;
> }
>
> ret = vb->vdev->config->find_vqs(vb->vdev, 4, vqs, callbacks, names,
> NULL, NULL);
> if (ret)
> return ret;
>
> vb->inflate_vq = vqs[0];
> vb->deflate_vq = vqs[1];
>
> if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_STATS_VQ)) {
> vb->stats_vq = vqs[2];
> /*
> * Prime this virtqueue with one buffer so the hypervisor
> can
> * use it to signal us later (it can't be broken yet!).
> */
> sg_init_one(&sg, vb->stats, sizeof(vb->stats));
> ret = virtqueue_add_outbuf(vb->stats_vq, &sg, 1, vb,
> GFP_KERNEL);
> if (ret) {
> dev_warn(&vb->vdev->dev, "%s: add stat_vq failed\n",
> __func__);
> return ret;
> }
> virtqueue_kick(vb->stats_vq);
> }
>
> if (virtio_has_feature(vb->vdev, VIRTIO_BALLOON_F_FREE_PAGE_VQ))
> vb->free_page_vq = vqs[3];
>
> return 0;
> }
>
>
> Btw, the QEMU side doesn't have an option to disable STATS_VQ currently, we
> may need to add that later.
>
> Best,
> Wei
why not
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