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From: Jason Wang <jasowang@redhat.com>
To: Tiwei Bie <tiwei.bie@intel.com>
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	virtualization@lists.linux-foundation.org,
	linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
	wexu@redhat.com, jfreimann@redhat.com
Subject: Re: [RFC v3 0/5] virtio: support packed ring
Date: Sat, 28 Apr 2018 10:45:52 +0800	[thread overview]
Message-ID: <e6629eba-3bfc-cbb7-8f74-9ffd8e178b57@redhat.com> (raw)
In-Reply-To: <20180427091234.n6vr3wcnbnqxd3so@debian>



On 2018年04月27日 17:12, Tiwei Bie wrote:
> On Fri, Apr 27, 2018 at 02:17:51PM +0800, Jason Wang wrote:
>> 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
> Yeah, that's my question too. It seems to be a requirement
> for driver that, the only change to the desc status that a
> driver can do during running is to mark the desc as avail,
> and any other changes to the desc status are not allowed.
> Similarly, the device can only mark the desc as used, and
> any other changes to the desc status are also not allowed.
> So the question is, are there such requirements?

Looks not, but I think we need clarify this in the spec.

Thanks

>
> Based on below contents in the spec:
>
> """
> Thus VIRTQ_DESC_F_AVAIL and VIRTQ_DESC_F_USED bits are different
> for an available descriptor and equal for a used descriptor.
>
> Note that this observation is mostly useful for sanity-checking
> as these are necessary but not sufficient conditions
> """
>
> It seems that, it's necessary for devices to check whether
> the AVAIL bit and USED bit are different.
>
> Best regards,
> Tiwei Bie

  reply	other threads:[~2018-04-28  2:46 UTC|newest]

Thread overview: 37+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-04-25  5:15 Tiwei Bie
2018-04-25  5:15 ` [RFC v3 1/5] virtio: add packed ring definitions Tiwei Bie
2018-04-25  5:15 ` [RFC v3 2/5] virtio_ring: support creating packed ring Tiwei Bie
2018-04-25  5:15 ` [RFC v3 3/5] virtio_ring: add packed ring support Tiwei Bie
2018-05-10  7:32   ` Jason Wang
2018-05-10  7:34     ` Jason Wang
2018-05-10  8:56       ` Tiwei Bie
2018-05-10  9:49         ` Jason Wang
2018-05-10 10:50           ` Tiwei Bie
2018-04-25  5:15 ` [RFC v3 4/5] virtio_ring: add event idx support in packed ring Tiwei Bie
2018-05-02  2:51   ` Jason Wang
2018-05-02  7:28     ` Tiwei Bie
2018-05-02 13:51       ` Michael S. Tsirkin
2018-05-02 15:12         ` Tiwei Bie
2018-05-02 15:42           ` Michael S. Tsirkin
2018-05-03  1:11             ` Tiwei Bie
2018-05-03  1:44               ` Michael S. Tsirkin
2018-05-03  2:09                 ` Tiwei Bie
2018-05-03  7:25                   ` Jason Wang
2018-05-03 13:54                     ` Tiwei Bie
2018-05-08  3:05                       ` Jason Wang
2018-05-08  5:40                         ` Jason Wang
2018-05-08  6:44                           ` Tiwei Bie
2018-05-08  7:16                             ` Jason Wang
2018-05-08  9:16                               ` Tiwei Bie
2018-05-08  9:34                                 ` Jason Wang
2018-05-08  9:44                                   ` Tiwei Bie
2018-05-09  3:43                                     ` Jason Wang
2018-05-16  5:01   ` Jason Wang
2018-05-16  5:55     ` Tiwei Bie
2018-04-25  5:15 ` [RFC v3 5/5] virtio_ring: enable " Tiwei Bie
2018-04-27  3:56 ` [RFC v3 0/5] virtio: support " Jason Wang
2018-04-27  4:18   ` Michael S. Tsirkin
2018-04-27  6:17     ` Jason Wang
2018-04-27  9:12       ` Tiwei Bie
2018-04-28  2:45         ` Jason Wang [this message]
2018-05-02  3:49 ` Jason Wang

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