Subject: Re: [RFC v5 3/5] virtio_ring: add packed ring support

From: Tiwei Bie <[email-masked]>
Date: Tue, 29 May 2018 13:11:25 +0800

On Tue, May 29, 2018 at 11:18:57AM +0800, Jason Wang wrote:
> On 2018年05月22日 16:16, Tiwei Bie wrote:
[...]
> > +static void detach_buf_packed(struct vring_virtqueue *vq,
> > +			      unsigned int id, void **ctx)
> > +{
> > +	struct vring_desc_state_packed *state;
> > +	struct vring_packed_desc *desc;
> > +	unsigned int i;
> > +	int *next;
> > +
> > +	/* Clear data ptr. */
> > +	vq->desc_state_packed[id].data = NULL;
> > +
> > +	next = &id;
> > +	for (i = 0; i < vq->desc_state_packed[id].num; i++) {
> > +		state = &vq->desc_state_packed[*next];
> > +		vring_unmap_state_packed(vq, state);
> > +		next = &vq->desc_state_packed[*next].next;
> 
> Have a short discussion with Michael. It looks like the only thing we care
> is DMA address and length here.

The length tracked by desc_state_packed[] is also necessary
when INDIRECT is used and the buffers are writable.

> 
> So a thought is to avoid using desc_state_packed() is vring_use_dma_api() is
> false? Probably need another id allocator or just use desc_state_packed for
> free id list.

I think it's a good suggestion. Thanks!

I won't track the addr/len/flags for non-indirect descs
when vring_use_dma_api() returns false.

> 
> > +	}
> > +
> > +	vq->vq.num_free += vq->desc_state_packed[id].num;
> > +	*next = vq->free_head;
> 
> Using pointer seems not elegant and unnecessary here. You can just track
> next in integer I think?

It's possible to use integer, but will need one more var
to track `prev` (desc_state_packed[prev].next == next in
this case).

> 
> > +	vq->free_head = id;
> > +
> > +	if (vq->indirect) {
> > +		u32 len;
> > +
> > +		/* Free the indirect table, if any, now that it's unmapped. */
> > +		desc = vq->desc_state_packed[id].indir_desc;
> > +		if (!desc)
> > +			return;
> > +
> > +		len = vq->desc_state_packed[id].len;
> > +		for (i = 0; i < len / sizeof(struct vring_packed_desc); i++)
> > +			vring_unmap_desc_packed(vq, &desc[i]);
> > +
> > +		kfree(desc);
> > +		vq->desc_state_packed[id].indir_desc = NULL;
> > +	} else if (ctx) {
> > +		*ctx = vq->desc_state_packed[id].indir_desc;
> > +	}
> >   }
> >   static inline bool more_used_packed(const struct vring_virtqueue *vq)
> >   {
> > -	return false;
> > +	u16 last_used, flags;
> > +	u8 avail, used;
> > +
> > +	last_used = vq->last_used_idx;
> > +	flags = virtio16_to_cpu(vq->vq.vdev,
> > +				vq->vring_packed.desc[last_used].flags);
> > +	avail = !!(flags & VRING_DESC_F_AVAIL(1));
> > +	used = !!(flags & VRING_DESC_F_USED(1));
> > +
> > +	return avail == used && used == vq->used_wrap_counter;
> 
> Spec does not check avail == used? So there's probably a bug in either of
> the two places.
> 
> In what condition that avail != used but used == vq_used_wrap_counter? A
> buggy device or device set the two bits in two writes?

Currently, spec doesn't forbid devices to set the status
bits as: avail != used but used == vq_used_wrap_counter.
So I think driver cannot assume this won't happen.

> >   }
[...]
> >   static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
> >   {
> > -	return 0;
> > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > +
> > +	START_USE(vq);
> > +
> > +	/* We optimistically turn back on interrupts, then check if there was
> > +	 * more to do. */
> > +
> > +	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> > +		virtio_wmb(vq->weak_barriers);
> 
> Missing comments for the barrier.

Will add some comments.

> 
> > +		vq->event_flags_shadow = VRING_EVENT_F_ENABLE;
> > +		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> > +							vq->event_flags_shadow);
> > +	}
> > +
> > +	END_USE(vq);
> > +	return vq->last_used_idx;
> >   }
> >   static bool virtqueue_poll_packed(struct virtqueue *_vq, unsigned last_used_idx)
> >   {
> > -	return false;
> > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > +	u8 avail, used;
> > +	u16 flags;
> > +
> > +	virtio_mb(vq->weak_barriers);
> > +	flags = virtio16_to_cpu(vq->vq.vdev,
> > +			vq->vring_packed.desc[last_used_idx].flags);
> > +	avail = !!(flags & VRING_DESC_F_AVAIL(1));
> > +	used = !!(flags & VRING_DESC_F_USED(1));
> > +
> > +	return avail == used && used == vq->used_wrap_counter;
> >   }
> >   static bool virtqueue_enable_cb_delayed_packed(struct virtqueue *_vq)
> >   {
> > -	return false;
> > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > +
> > +	START_USE(vq);
> > +
> > +	/* We optimistically turn back on interrupts, then check if there was
> > +	 * more to do. */
> > +
> > +	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> > +		virtio_wmb(vq->weak_barriers);
> 
> Need comments for the barrier.

Will add some comments.

Best regards,
Tiwei Bie