Re: [Qemu-devel] [PATCH for-4.2 v10 03/15] virtio-iommu: Add skeleton

[Peter Xu <peterx@redhat.com>]

On Thu, Aug 29, 2019 at 02:18:42PM +0200, Auger Eric wrote:
> Hi Peter,
> 
> First of all, please forgive me for the delay.
> On 8/15/19 3:54 PM, Peter Xu wrote:
> > On Tue, Jul 30, 2019 at 07:21:25PM +0200, Eric Auger wrote:
> >> +static void virtio_iommu_handle_command(VirtIODevice *vdev, VirtQueue *vq)
> >> +{
> >> +    VirtIOIOMMU *s = VIRTIO_IOMMU(vdev);
> >> +    struct virtio_iommu_req_head head;
> >> +    struct virtio_iommu_req_tail tail;
> > 
> > [1]
> > 
> >> +    VirtQueueElement *elem;
> >> +    unsigned int iov_cnt;
> >> +    struct iovec *iov;
> >> +    size_t sz;
> >> +
> >> +    for (;;) {
> >> +        elem = virtqueue_pop(vq, sizeof(VirtQueueElement));
> >> +        if (!elem) {
> >> +            return;
> >> +        }
> >> +
> >> +        if (iov_size(elem->in_sg, elem->in_num) < sizeof(tail) ||
> >> +            iov_size(elem->out_sg, elem->out_num) < sizeof(head)) {
> >> +            virtio_error(vdev, "virtio-iommu bad head/tail size");
> >> +            virtqueue_detach_element(vq, elem, 0);
> >> +            g_free(elem);
> >> +            break;
> >> +        }
> >> +
> >> +        iov_cnt = elem->out_num;
> >> +        iov = g_memdup(elem->out_sg, sizeof(struct iovec) * elem->out_num);
> > 
> > Could I ask why memdup is needed here?
> Indeed I don't think it is needed and besides iov is not freed!
> 
> I got inspired from hw/net/virtio-net.c. To be honest I don't get why
> the g_memdup is needed there either. The out_sg gets duplicated and
> commands work on the duplicated data and not in place.

Oh true, I found that it's because of calling of iov_discard_front().
Please have a look at 771b6ed37e3.  Though it seems to me that
virtio-iommu does not truncate iovs so it should not be needed.

> > 
> >> +        sz = iov_to_buf(iov, iov_cnt, 0, &head, sizeof(head));
> >> +        if (unlikely(sz != sizeof(head))) {
> >> +            tail.status = VIRTIO_IOMMU_S_DEVERR;
> > 
> > Do you need to zero the reserved bits to make sure it won't contain
> > garbage?  Same question to below uses of tail.
> yes. I initialized tail.
> > 
> >> +            goto out;
> >> +        }
> >> +        qemu_mutex_lock(&s->mutex);
> >> +        switch (head.type) {
> >> +        case VIRTIO_IOMMU_T_ATTACH:
> >> +            tail.status = virtio_iommu_handle_attach(s, iov, iov_cnt);
> >> +            break;
> >> +        case VIRTIO_IOMMU_T_DETACH:
> >> +            tail.status = virtio_iommu_handle_detach(s, iov, iov_cnt);
> >> +            break;
> >> +        case VIRTIO_IOMMU_T_MAP:
> >> +            tail.status = virtio_iommu_handle_map(s, iov, iov_cnt);
> >> +            break;
> >> +        case VIRTIO_IOMMU_T_UNMAP:
> >> +            tail.status = virtio_iommu_handle_unmap(s, iov, iov_cnt);
> >> +            break;
> >> +        default:
> >> +            tail.status = VIRTIO_IOMMU_S_UNSUPP;
> >> +        }
> >> +        qemu_mutex_unlock(&s->mutex);
> >> +
> >> +out:
> >> +        sz = iov_from_buf(elem->in_sg, elem->in_num, 0,
> >> +                          &tail, sizeof(tail));
> >> +        assert(sz == sizeof(tail));
> >> +
> >> +        virtqueue_push(vq, elem, sizeof(tail));
> > 
> > s/tail/head/ (though they are the same size)?
> That's unclear to me. Similarly when checking against virtio-net.c, the
> element is pushed back to the used ring and len is set to the size of
> the status with:
> 
> /*
>  * Control virtqueue data structures
>  *
>  * The control virtqueue expects a header in the first sg entry
>  * and an ack/status response in the last entry.  Data for the
>  * command goes in between.
>  */

I was referencing the balloon code when reading the patch, e.g.,
virtio_balloon_handle_output().  Though after I read more carefully I
see that other places are using it as you described.  Now I tend to
agree with you, because virtqueue_push() who calls
virtqueue_unmap_sg() used the len to unmap in_sg[] rather than
out_sg[].  So please ignore my previous comment.

(then I'm not sure whether the usage in the balloon code was correct
 now...)

> > 
> >> +        virtio_notify(vdev, vq);
> >> +        g_free(elem);
> >> +    }
> >> +}
> > 
> > [...]
> > 
> >> +static void virtio_iommu_set_features(VirtIODevice *vdev, uint64_t val)
> >> +{
> >> +    VirtIOIOMMU *dev = VIRTIO_IOMMU(vdev);
> >> +
> >> +    dev->acked_features = val;
> >> +    trace_virtio_iommu_set_features(dev->acked_features);
> >> +}
> >> +
> >> +static const VMStateDescription vmstate_virtio_iommu_device = {
> >> +    .name = "virtio-iommu-device",
> >> +    .unmigratable = 1,
> > 
> > Curious, is there explicit reason to not support migration from the
> > first version? :)
> The state is made of red black trees, lists. For the former there is no
> VMSTATE* ready. I am working on it but I think this should be handled
> separately

Fair enough.  Would you mind to add a similar comment above
unmigratable?

Thanks,

-- 
Peter Xu