linux-kernel - Re: [PATCH net-next V2 6/8] vhost: packed ring support

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Message-ID: <0f3827e5-a7fa-e54a-725d-7726e90333b8@redhat.com>
Date:   Wed, 17 Oct 2018 14:54:22 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Maxime Coquelin <maxime.coquelin@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Tiwei Bie <tiwei.bie@...el.com>
Cc:     kvm@...r.kernel.org, virtualization@...ts.linux-foundation.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        wexu@...hat.com, jfreimann@...hat.com
Subject: Re: [PATCH net-next V2 6/8] vhost: packed ring support


On 2018/10/16 下午9:58, Maxime Coquelin wrote:
>
> On 10/15/2018 04:22 AM, Jason Wang wrote:
>>
>>
>> On 2018年10月13日 01:23, Michael S. Tsirkin wrote:
>>> On Fri, Oct 12, 2018 at 10:32:44PM +0800, Tiwei Bie wrote:
>>>> On Mon, Jul 16, 2018 at 11:28:09AM +0800, Jason Wang wrote:
>>>> [...]
>>>>> @@ -1367,10 +1397,48 @@ long vhost_vring_ioctl(struct vhost_dev 
>>>>> *d, unsigned int ioctl, void __user *arg
>>>>>           vq->last_avail_idx = s.num;
>>>>>           /* Forget the cached index value. */
>>>>>           vq->avail_idx = vq->last_avail_idx;
>>>>> +        if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED)) {
>>>>> +            vq->last_avail_wrap_counter = wrap_counter;
>>>>> +            vq->avail_wrap_counter = vq->last_avail_wrap_counter;
>>>>> +        }
>>>>>           break;
>>>>>       case VHOST_GET_VRING_BASE:
>>>>>           s.index = idx;
>>>>>           s.num = vq->last_avail_idx;
>>>>> +        if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED))
>>>>> +            s.num |= vq->last_avail_wrap_counter << 31;
>>>>> +        if (copy_to_user(argp, &s, sizeof(s)))
>>>>> +            r = -EFAULT;
>>>>> +        break;
>>>>> +    case VHOST_SET_VRING_USED_BASE:
>>>>> +        /* Moving base with an active backend?
>>>>> +         * You don't want to do that.
>>>>> +         */
>>>>> +        if (vq->private_data) {
>>>>> +            r = -EBUSY;
>>>>> +            break;
>>>>> +        }
>>>>> +        if (copy_from_user(&s, argp, sizeof(s))) {
>>>>> +            r = -EFAULT;
>>>>> +            break;
>>>>> +        }
>>>>> +        if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED)) {
>>>>> +            wrap_counter = s.num >> 31;
>>>>> +            s.num &= ~(1 << 31);
>>>>> +        }
>>>>> +        if (s.num > 0xffff) {
>>>>> +            r = -EINVAL;
>>>>> +            break;
>>>>> +        }
>>>> Do we want to put wrap_counter at bit 15?
>>> I think I second that - seems to be consistent with
>>> e.g. event suppression structure and the proposed
>>> extension to driver notifications.
>>
>> Ok, I assumes packed virtqueue support 64K but looks not. I can 
>> change it to bit 15 and GET_VRING_BASE need to be changed as well.
>>
>>>
>>>
>>>> If put wrap_counter at bit 31, the check (s.num > 0xffff)
>>>> won't be able to catch the illegal index 0x8000~0xffff for
>>>> packed ring.
>>>>
>>
>> Do we need to clarify this in the spec?
>>
>>>>> +        vq->last_used_idx = s.num;
>>>>> +        if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED))
>>>>> +            vq->last_used_wrap_counter = wrap_counter;
>>>>> +        break;
>>>>> +    case VHOST_GET_VRING_USED_BASE:
>>>> Do we need the new VHOST_GET_VRING_USED_BASE and
>>>> VHOST_SET_VRING_USED_BASE ops?
>>>>
>>>> We are going to merge below series in DPDK:
>>>>
>>>> http://patches.dpdk.org/patch/45874/
>>>>
>>>> We may need to reach an agreement first.
>>
>> If we agree that 64K virtqueue won't be supported, I'm ok with either.
>
> I'm fine to put wrap_counter at bit 15.
> I will post a new version of the DPDK series soon.
>
>> Btw the code assumes used_wrap_counter is equal to avail_wrap_counter 
>> which looks wrong?
>
> For split ring, we used to set the last_used_idx to the same value as
> last_avail_idx as VHOST_USER_GET_VRING_BASE cannot be called while the
> ring is being processed, so their value is always the same at the time
> the request is handled.


I may miss something, but it looks to me we should sync last_used_idx 
from used_idx.

Thanks


>
>
> I kept the same behavior for packed ring, and so the wrap counter have
> to be the same.
>
> Regards,
> Maxime
>
>> Thanks
>>
>>>>
>>>>> +        s.index = idx;
>>>>> +        s.num = vq->last_used_idx;
>>>>> +        if (vhost_has_feature(vq, VIRTIO_F_RING_PACKED))
>>>>> +            s.num |= vq->last_used_wrap_counter << 31;
>>>>>           if (copy_to_user(argp, &s, sizeof s))
>>>>>               r = -EFAULT;
>>>>>           break;
>>>> [...]
>>