Re: [RFC v2 5/7] virtio: unmask F_NEXT flag in desc_extra

[Guo Zhi <qtxuning1999@sjtu.edu.cn>]

----- Original Message -----
> From: "Xuan Zhuo" <xuanzhuo@linux.alibaba.com>
> To: "Guo Zhi" <qtxuning1999@sjtu.edu.cn>
> Cc: "netdev" <netdev@vger.kernel.org>, "linux-kernel" <linux-kernel@vger.kernel.org>, "kvm list" <kvm@vger.kernel.org>,
> "virtualization" <virtualization@lists.linux-foundation.org>, "Guo Zhi" <qtxuning1999@sjtu.edu.cn>, "eperezma"
> <eperezma@redhat.com>, "jasowang" <jasowang@redhat.com>, "sgarzare" <sgarzare@redhat.com>, "Michael Tsirkin"
> <mst@redhat.com>
> Sent: Thursday, August 18, 2022 11:05:37 AM
> Subject: Re: [RFC v2 5/7] virtio: unmask F_NEXT flag in desc_extra

> On Wed, 17 Aug 2022 21:57:16 +0800, Guo Zhi <qtxuning1999@sjtu.edu.cn> wrote:
>> We didn't unmask F_NEXT flag in desc_extra in the end of a chain,
>> unmask it so that we can access desc_extra to get next information.
> 
> I think we should state the purpose of this.
> 
I have to unmask F_NEXT flag in desc_extra in this series, because if in order
negotiated, the driver has to iterate the descriptor chain to get chain length
from desc_extra. (The reason why we should use desc_extra is that descs may be
changed by malicious hypervisors, https://lkml.org/lkml/2022/7/26/224).

>>
>> Signed-off-by: Guo Zhi <qtxuning1999@sjtu.edu.cn>
>> ---
>>  drivers/virtio/virtio_ring.c | 6 ++++--
>>  1 file changed, 4 insertions(+), 2 deletions(-)
>>
>> diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
>> index a5ec724c01d8..1c1b3fa376a2 100644
>> --- a/drivers/virtio/virtio_ring.c
>> +++ b/drivers/virtio/virtio_ring.c
>> @@ -567,7 +567,7 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>>  	}
>>  	/* Last one doesn't continue. */
>>  	desc[prev].flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
>> -	if (!indirect && vq->use_dma_api)
>> +	if (!indirect)
>>  		vq->split.desc_extra[prev & (vq->split.vring.num - 1)].flags &=
>>  			~VRING_DESC_F_NEXT;
>>
>> @@ -584,6 +584,8 @@ static inline int virtqueue_add_split(struct virtqueue *_vq,
>>  					 total_sg * sizeof(struct vring_desc),
>>  					 VRING_DESC_F_INDIRECT,
>>  					 false);
>> +		vq->split.desc_extra[head & (vq->split.vring.num - 1)].flags &=
>> +			~VRING_DESC_F_NEXT;
> 
> This seems unnecessary.
> 
>>  	}
>>
>>  	/* We're using some buffers from the free list. */
>> @@ -693,7 +695,7 @@ static void detach_buf_split(struct vring_virtqueue *vq,
>> unsigned int head,
>>  	/* Put back on free list: unmap first-level descriptors and find end */
>>  	i = head;
>>
>> -	while (vq->split.vring.desc[i].flags & nextflag) {
>> +	while (vq->split.desc_extra[i].flags & nextflag) {
> 
> nextflag is __virtio16
> 
> You can use VRING_DESC_F_NEXT directly.
> 
> Thanks.

Sorry for the mistake, I will fix it.

> 
>>  		vring_unmap_one_split(vq, i);
>>  		i = vq->split.desc_extra[i].next;
>>  		vq->vq.num_free++;
>> --
>> 2.17.1
>>