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

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Date:   Tue, 16 Oct 2018 15:58:22 +0200
From:   Maxime Coquelin <maxime.coquelin@...hat.com>
To:     Jason Wang <jasowang@...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 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 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;
>>> [...]
>