Re: [RFC v3 0/5] virtio: support packed ring

[Jason Wang <jasowang@redhat.com>]

On 2018年04月27日 12:18, Michael S. Tsirkin wrote:
> On Fri, Apr 27, 2018 at 11:56:05AM +0800, Jason Wang wrote:
>> On 2018年04月25日 13:15, Tiwei Bie wrote:
>>> Hello everyone,
>>>
>>> This RFC implements packed ring support in virtio driver.
>>>
>>> Some simple functional tests have been done with Jason's
>>> packed ring implementation in vhost:
>>>
>>> https://lkml.org/lkml/2018/4/23/12
>>>
>>> Both of ping and netperf worked as expected (with EVENT_IDX
>>> disabled). But there are below known issues:
>>>
>>> 1. Reloading the guest driver will break the Tx/Rx;
>> Will have a look at this issue.
>>
>>> 2. Zeroing the flags when detaching a used desc will
>>>      break the guest -> host path.
>> I still think zeroing flags is unnecessary or even a bug. At host, I track
>> last observed avail wrap counter and detect avail like (what is suggested in
>> the example code in the spec):
>>
>> static bool desc_is_avail(struct vhost_virtqueue *vq, __virtio16 flags)
>> {
>>         bool avail = flags & cpu_to_vhost16(vq, DESC_AVAIL);
>>
>>         return avail == vq->avail_wrap_counter;
>> }
>>
>> So zeroing wrap can not work with this obviously.
>>
>> Thanks
> I agree. I think what one should do is flip the available bit.
>

But is this flipping a must?

Thanks