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

[Auger Eric]

Hi Peter,
On 8/30/19 3:26 AM, Peter Xu wrote:
> 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.

thanks for the sha1. indeed virtio-iommu does not use iov_discard_front
so I shouldn't need it.
> 
>>>
>>>> +        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.

OK
> 
> (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?
sure

Thanks!

Eric
> 
> Thanks,
>