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

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Date:   Mon, 15 Oct 2018 10:51:06 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Tiwei Bie <tiwei.bie@...el.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, wexu@...hat.com,
        jfreimann@...hat.com, maxime.coquelin@...hat.com
Subject: Re: [PATCH net-next V2 6/8] vhost: packed ring support



On 2018年10月15日 10:43, Michael S. Tsirkin wrote:
> On Mon, Oct 15, 2018 at 10:22:33AM +0800, 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?
> Isn't this all internal vhost stuff?

I meant the illegal index 0x8000-0xffff.

>
>>>>> +		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.
> Well the spec says right at the beginning:
>
> Packed virtqueues support up to 2 15 entries each.

Ok. I get it.

Then I can change vhost to match what dpdk did.

Thanks

>
>
>> Btw the code assumes used_wrap_counter is equal to avail_wrap_counter which
>> looks wrong?
>>
>> 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;
>>>> [...]