Re: [RFC 4/5] virtio: get desc id in order

[Guo Zhi]

On 2022/7/26 16:07, Jason Wang wrote:
> 在 2022/7/21 16:43, Guo Zhi 写道:
> > If in order feature negotiated, we can skip the used ring to get
> > buffer's desc id sequentially.
>
>
> Let's rename the patch to something like "in order support for 
> virtio_ring"
>
>
> > Signed-off-by: Guo Zhi <qtxuning1999@xxxxxxxxxxx>
> > ---
> >   drivers/virtio/virtio_ring.c | 37 ++++++++++++++++++++++++++++--------
> >   1 file changed, 29 insertions(+), 8 deletions(-)
>
>
> I don't see packed support in this patch, we need to implement that.
It will be implemented later.
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index a5ec724c0..4d57a4edc 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -144,6 +144,9 @@ struct vring_virtqueue {
> >               /* DMA address and size information */
> >               dma_addr_t queue_dma_addr;
> >               size_t queue_size_in_bytes;
> > +
> > +            /* In order feature batch begin here */
> > +            u16 next_batch_desc_begin;
> >           } split;
> >             /* Available for packed ring */
> > @@ -700,8 +703,10 @@ static void detach_buf_split(struct 
> > vring_virtqueue *vq, unsigned int head,
> >       }
> >         vring_unmap_one_split(vq, i);
> > -    vq->split.desc_extra[i].next = vq->free_head;
> > -    vq->free_head = head;
> > +    if (!virtio_has_feature(vq->vq.vdev, VIRTIO_F_IN_ORDER)) {
> > +        vq->split.desc_extra[i].next = vq->free_head;
> > +        vq->free_head = head;
> > +    }
>
>
> Let's add a comment to explain why we don't need anything if in order 
> is neogitated.
LGTM.
> >         /* Plus final descriptor */
> >       vq->vq.num_free++;
> > @@ -743,7 +748,8 @@ static void *virtqueue_get_buf_ctx_split(struct 
> > virtqueue *_vq,
> >   {
> >       struct vring_virtqueue *vq = to_vvq(_vq);
> >       void *ret;
> > -    unsigned int i;
> > +    __virtio16 nextflag = cpu_to_virtio16(vq->vq.vdev, 
> > VRING_DESC_F_NEXT);
> > +    unsigned int i, j;
> >       u16 last_used;
> >         START_USE(vq);
> > @@ -762,11 +768,24 @@ static void *virtqueue_get_buf_ctx_split(struct 
> > virtqueue *_vq,
> >       /* Only get used array entries after they have been exposed by 
> > host. */
> >       virtio_rmb(vq->weak_barriers);
> >   -    last_used = (vq->last_used_idx & (vq->split.vring.num - 1));
> > -    i = virtio32_to_cpu(_vq->vdev,
> > -            vq->split.vring.used->ring[last_used].id);
> > -    *len = virtio32_to_cpu(_vq->vdev,
> > -            vq->split.vring.used->ring[last_used].len);
> > +    if (virtio_has_feature(_vq->vdev, VIRTIO_F_IN_ORDER)) {
> > +        /* Skip used ring and get used desc in order*/
> > +        i = vq->split.next_batch_desc_begin;
> > +        j = i;
> > +        while (vq->split.vring.desc[j].flags & nextflag)
>
>
> Let's don't depend on the descriptor ring which is under the control 
> of the malicious hypervisor.
>
> Let's use desc_extra that is not visible by the hypervisor. More can 
> be seen in this commit:
>
> 72b5e8958738 ("virtio-ring: store DMA metadata in desc_extra for split 
> virtqueue")
LGTM, I will use desc_extra in new version patch.
> > +            j = (j + 1) % vq->split.vring.num;
> > +        /* move to next */
> > +        j = (j + 1) % vq->split.vring.num;
> > +        vq->split.next_batch_desc_begin = j;
>
>
> I'm not sure I get the logic here, basically I think we should check 
> buffer instead of descriptor here.

Because the vq->last_used_idx != vq->split.vring.used->idx, So the 
virtio driver know these has at least one used descriptor. the 
descriptor's id is vq->split.next_batch_desc_begin because of in order. 
Then we have to traverse the descriptor chain and point 
vq->split.next_batch_desc_begin to next used descriptor.

Thanks.

> So if vring.used->ring[last_used].id != last_used, we know all 
> [last_used, vring.used->ring[last_used].id] have been used in a batch?
>
>
> > +
> > +        /* TODO: len of buffer */
>
>
> So spec said:
>
> "
>
> The skipped buffers (for which no used ring entry was written) are 
> assumed to have been used (read or written) by the device completely.
>
>
> "
>
> Thanks
The driver will need len in used ring to get buffer size. However in 
order will not write len of each buffer in used ring. So I will tried 
pass len of buffer in device header.
> > +    } else {
> > +        last_used = (vq->last_used_idx & (vq->split.vring.num - 1));
> > +        i = virtio32_to_cpu(_vq->vdev,
> > + vq->split.vring.used->ring[last_used].id);
> > +        *len = virtio32_to_cpu(_vq->vdev,
> > + vq->split.vring.used->ring[last_used].len);
> > +    }
> >         if (unlikely(i >= vq->split.vring.num)) {
> >           BAD_RING(vq, "id %u out of range\n", i);
> > @@ -2234,6 +2253,8 @@ struct virtqueue 
> > *__vring_new_virtqueue(unsigned int index,
> >       vq->split.avail_flags_shadow = 0;
> >       vq->split.avail_idx_shadow = 0;
> >   +    vq->split.next_batch_desc_begin = 0;
> > +
> >       /* No callback?  Tell other side not to bother us. */
> >       if (!callback) {
> >           vq->split.avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;