Subject: Re: [RFC v2] virtio: support packed ring

From: "Michael S. Tsirkin" <[email-masked]>
Date: Tue, 17 Apr 2018 05:17:41 +0300

On Tue, Apr 17, 2018 at 10:11:58AM +0800, Jason Wang wrote:
> 
> 
> On 2018年04月13日 15:15, Tiwei Bie wrote:
> > On Fri, Apr 13, 2018 at 12:30:24PM +0800, Jason Wang wrote:
> > > On 2018年04月01日 22:12, Tiwei Bie wrote:
> > > > Hello everyone,
> > > > 
> > > > This RFC implements packed ring support for virtio driver.
> > > > 
> > > > The code was tested with DPDK vhost (testpmd/vhost-PMD) implemented
> > > > by Jens at http://dpdk.org/ml/archives/dev/2018-January/089417.html
> > > > Minor changes are needed for the vhost code, e.g. to kick the guest.
> > > > 
> > > > TODO:
> > > > - Refinements and bug fixes;
> > > > - Split into small patches;
> > > > - Test indirect descriptor support;
> > > > - Test/fix event suppression support;
> > > > - Test devices other than net;
> > > > 
> > > > RFC v1 -> RFC v2:
> > > > - Add indirect descriptor support - compile test only;
> > > > - Add event suppression supprt - compile test only;
> > > > - Move vring_packed_init() out of uapi (Jason, MST);
> > > > - Merge two loops into one in virtqueue_add_packed() (Jason);
> > > > - Split vring_unmap_one() for packed ring and split ring (Jason);
> > > > - Avoid using '%' operator (Jason);
> > > > - Rename free_head -> next_avail_idx (Jason);
> > > > - Add comments for virtio_wmb() in virtqueue_add_packed() (Jason);
> > > > - Some other refinements and bug fixes;
> > > > 
> > > > Thanks!
> > > > 
> > > > Signed-off-by: Tiwei Bie <[email-masked]>
> > > > ---
> > > >    drivers/virtio/virtio_ring.c       | 1094 +++++++++++++++++++++++++++++-------
> > > >    include/linux/virtio_ring.h        |    8 +-
> > > >    include/uapi/linux/virtio_config.h |   12 +-
> > > >    include/uapi/linux/virtio_ring.h   |   61 ++
> > > >    4 files changed, 980 insertions(+), 195 deletions(-)
> > [...]
> > > > +static struct vring_packed_desc *alloc_indirect_packed(struct virtqueue *_vq,
> > > > +						       unsigned int total_sg,
> > > > +						       gfp_t gfp)
> > > > +{
> > > > +	struct vring_packed_desc *desc;
> > > > +
> > > > +	/*
> > > > +	 * We require lowmem mappings for the descriptors because
> > > > +	 * otherwise virt_to_phys will give us bogus addresses in the
> > > > +	 * virtqueue.
> > > > +	 */
> > > > +	gfp &= ~__GFP_HIGHMEM;
> > > > +
> > > > +	desc = kmalloc(total_sg * sizeof(struct vring_packed_desc), gfp);
> > > Can we simply check vq->packed here to avoid duplicating helpers?
> > Then it would be something like this:
> > 
> > static void *alloc_indirect(struct virtqueue *_vq, unsigned int total_sg,
> > 			    gfp_t gfp)
> > {
> > 	struct vring_virtqueue *vq = to_vvq(_vq);
> > 	void *data;
> > 
> > 	/*
> > 	 * We require lowmem mappings for the descriptors because
> > 	 * otherwise virt_to_phys will give us bogus addresses in the
> > 	 * virtqueue.
> > 	 */
> > 	gfp &= ~__GFP_HIGHMEM;
> > 
> > 	if (vq->packed) {
> > 		data = kmalloc(total_sg * sizeof(struct vring_packed_desc),
> > 				gfp);
> > 		if (!data)
> > 			return NULL;
> > 	} else {
> > 		struct vring_desc *desc;
> > 		unsigned int i;
> > 
> > 		desc = kmalloc(total_sg * sizeof(struct vring_desc), gfp);
> > 		if (!desc)
> > 			return NULL;
> > 
> > 		for (i = 0; i < total_sg; i++)
> > 			desc[i].next = cpu_to_virtio16(_vq->vdev, i + 1);
> > 
> > 		data = desc;
> > 	}
> > 
> > 	return data;
> > }
> > 
> > I would prefer to have two simpler helpers (and to the callers,
> > it's already very clear about which one they should call), i.e.
> > the current implementation:
> > 
> > static struct vring_packed_desc *alloc_indirect_packed(struct virtqueue *_vq,
> > 						       unsigned int total_sg,
> > 						       gfp_t gfp)
> > {
> > 	struct vring_packed_desc *desc;
> > 
> > 	/*
> > 	 * We require lowmem mappings for the descriptors because
> > 	 * otherwise virt_to_phys will give us bogus addresses in the
> > 	 * virtqueue.
> > 	 */
> > 	gfp &= ~__GFP_HIGHMEM;
> > 
> > 	desc = kmalloc(total_sg * sizeof(struct vring_packed_desc), gfp);
> > 
> > 	return desc;
> > }
> > 
> > static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
> > 					       unsigned int total_sg,
> > 					       gfp_t gfp)
> > {
> > 	struct vring_desc *desc;
> > 	unsigned int i;
> > 
> > 	/*
> > 	 * We require lowmem mappings for the descriptors because
> > 	 * otherwise virt_to_phys will give us bogus addresses in the
> > 	 * virtqueue.
> > 	 */
> > 	gfp &= ~__GFP_HIGHMEM;
> > 
> > 	desc = kmalloc(total_sg * sizeof(struct vring_desc), gfp);
> > 	if (!desc)
> > 		return NULL;
> > 
> > 	for (i = 0; i < total_sg; i++)
> > 		desc[i].next = cpu_to_virtio16(_vq->vdev, i + 1);
> > 	return desc;
> > }
> 
> Yeah, I miss that split version needs a desc list.
> 
> > 
> > > > +
> > > > +	return desc;
> > > > +}
> > [...]
> > > > +static inline int virtqueue_add_packed(struct virtqueue *_vq,
> > > > +				       struct scatterlist *sgs[],
> > > > +				       unsigned int total_sg,
> > > > +				       unsigned int out_sgs,
> > > > +				       unsigned int in_sgs,
> > > > +				       void *data,
> > > > +				       void *ctx,
> > > > +				       gfp_t gfp)
> > > > +{
> > > > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > > > +	struct vring_packed_desc *desc;
> > > > +	struct scatterlist *sg;
> > > > +	unsigned int i, n, descs_used, uninitialized_var(prev), err_idx;
> > > > +	__virtio16 uninitialized_var(head_flags), flags;
> > > > +	int head, wrap_counter;
> > > > +	bool indirect;
> > > > +
> > > > +	START_USE(vq);
> > > > +
> > > > +	BUG_ON(data == NULL);
> > > > +	BUG_ON(ctx && vq->indirect);
> > > > +
> > > > +	if (unlikely(vq->broken)) {
> > > > +		END_USE(vq);
> > > > +		return -EIO;
> > > > +	}
> > > > +
> > > > +#ifdef DEBUG
> > > > +	{
> > > > +		ktime_t now = ktime_get();
> > > > +
> > > > +		/* No kick or get, with .1 second between?  Warn. */
> > > > +		if (vq->last_add_time_valid)
> > > > +			WARN_ON(ktime_to_ms(ktime_sub(now, vq->last_add_time))
> > > > +					    > 100);
> > > > +		vq->last_add_time = now;
> > > > +		vq->last_add_time_valid = true;
> > > > +	}
> > > > +#endif
> > > > +
> > > > +	BUG_ON(total_sg == 0);
> > > > +
> > > > +	head = vq->next_avail_idx;
> > > > +	wrap_counter = vq->wrap_counter;
> > > > +
> > > > +	/* If the host supports indirect descriptor tables, and we have multiple
> > > > +	 * buffers, then go indirect. FIXME: tune this threshold */
> > > > +	if (vq->indirect && total_sg > 1 && vq->vq.num_free)
> > > Let's introduce a helper like virtqueue_need_indirect() to avoid duplicating
> > > codes and FIXME.
> > Okay.
> > 
> > > > +		desc = alloc_indirect_packed(_vq, total_sg, gfp);
> > > > +	else {
> > > > +		desc = NULL;
> > > > +		WARN_ON_ONCE(total_sg > vq->vring_packed.num && !vq->indirect);
> > > > +	}
> > > > +
> > > > +	if (desc) {
> > > > +		/* Use a single buffer which doesn't continue */
> > > > +		indirect = true;
> > > > +		/* Set up rest to use this indirect table. */
> > > > +		i = 0;
> > > > +		descs_used = 1;
> > > > +	} else {
> > > > +		indirect = false;
> > > > +		desc = vq->vring_packed.desc;
> > > > +		i = head;
> > > > +		descs_used = total_sg;
> > > > +	}
> > > > +
> > > > +	if (vq->vq.num_free < descs_used) {
> > > > +		pr_debug("Can't add buf len %i - avail = %i\n",
> > > > +			 descs_used, vq->vq.num_free);
> > > > +		/* FIXME: for historical reasons, we force a notify here if
> > > > +		 * there are outgoing parts to the buffer.  Presumably the
> > > > +		 * host should service the ring ASAP. */
> > > > +		if (out_sgs)
> > > > +			vq->notify(&vq->vq);
> > > > +		if (indirect)
> > > > +			kfree(desc);
> > > > +		END_USE(vq);
> > > > +		return -ENOSPC;
> > > > +	}
> > > > +
> > > > +	for (n = 0; n < out_sgs + in_sgs; n++) {
> > > > +		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> > > > +			dma_addr_t addr = vring_map_one_sg(vq, sg, n < out_sgs ?
> > > > +						DMA_TO_DEVICE : DMA_FROM_DEVICE);
> > > > +			if (vring_mapping_error(vq, addr))
> > > > +				goto unmap_release;
> > > > +
> > > > +			flags = cpu_to_virtio16(_vq->vdev, VRING_DESC_F_NEXT |
> > > > +					(n < out_sgs ? 0 : VRING_DESC_F_WRITE) |
> > > > +					VRING_DESC_F_AVAIL(vq->wrap_counter) |
> > > > +					VRING_DESC_F_USED(!vq->wrap_counter));
> > > > +			if (!indirect && i == head)
> > > > +				head_flags = flags;
> > > > +			else
> > > > +				desc[i].flags = flags;
> > > > +
> > > > +			desc[i].addr = cpu_to_virtio64(_vq->vdev, addr);
> > > > +			desc[i].len = cpu_to_virtio32(_vq->vdev, sg->length);
> > > > +			desc[i].id = cpu_to_virtio32(_vq->vdev, head);
> > > Similar to V1, we only need this for the last descriptor.
> > Okay, will just set it for the last desc.
> > 
> > > > +			prev = i;
> > > It looks to me there's no need to track prev inside the loop here.
> > > 
> > > > +			i++;
> > > > +			if (!indirect && i >= vq->vring_packed.num) {
> > > > +				i = 0;
> > > > +				vq->wrap_counter ^= 1;
> > > > +			}
> > > > +		}
> > > > +	}
> > > > +	/* Last one doesn't continue. */
> > > > +	if (total_sg == 1)
> > > > +		head_flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
> > > > +	else
> > > > +		desc[prev].flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
> > > The only case when prev != i - 1 is i == 0, we can add a if here.
> > It's just a mirror of the existing implementation in split ring.
> > It seems that split ring implementation needs this just because
> > it's much harder for it to find the prev, which is not true for
> > packed ring. So I'll take your suggestion. Thanks!
> > 
> > [...]
> > > > +static bool virtqueue_kick_prepare_packed(struct virtqueue *_vq)
> > > > +{
> > > > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > > > +	u16 new, old, off_wrap;
> > > > +	bool needs_kick;
> > > > +
> > > > +	START_USE(vq);
> > > > +	/* We need to expose the new flags value before checking notification
> > > > +	 * suppressions. */
> > > > +	virtio_mb(vq->weak_barriers);
> > > > +
> > > > +	old = vq->next_avail_idx - vq->num_added;
> > > > +	new = vq->next_avail_idx;
> > > > +	vq->num_added = 0;
> > > > +
> > > > +#ifdef DEBUG
> > > > +	if (vq->last_add_time_valid) {
> > > > +		WARN_ON(ktime_to_ms(ktime_sub(ktime_get(),
> > > > +					      vq->last_add_time)) > 100);
> > > > +	}
> > > > +	vq->last_add_time_valid = false;
> > > > +#endif
> > > > +
> > > > +	off_wrap = virtio16_to_cpu(_vq->vdev, vq->vring_packed.device->off_wrap);
> > > > +
> > > > +	if (vq->event) {
> > > It looks to me we should examine RING_EVENT_FLAGS_DESC in desc_event_flags
> > > instead of vq->event here. Spec does not forces to use evenf_off and
> > > event_wrap if event index is enabled.
> > > 
> > > > +		// FIXME: fix this!
> > > > +		needs_kick = ((off_wrap >> 15) == vq->wrap_counter) &&
> > > > +			     vring_need_event(off_wrap & ~(1<<15), new, old);
> > > Why need a & here?
> > Because wrap_counter (the most significant bit in off_wrap)
> > isn't part of the index.
> > 
> > > > +	} else {
> > > Need a smp_rmb() to make sure desc_event_flags was checked before flags.
> > I don't get your point, if my understanding is correct,
> > desc_event_flags is vq->vring_packed.device->flags. So
> > what's the "flags"?
> 
> Sorry, I mean we need check device.flags before off_warp. So it needs an
> smp_rmb() in the middle.

It's best to just read all flags atomically as u32.

> It looks to me there's no guarantee that
> VRING_EVENT_F_DESC is set if event index is supported.
> 
> > 
> > > > +		needs_kick = (vq->vring_packed.device->flags !=
> > > > +			      cpu_to_virtio16(_vq->vdev, VRING_EVENT_F_DISABLE));
> > > > +	}
> > > > +	END_USE(vq);
> > > > +	return needs_kick;
> > > > +}
> > [...]
> > > > +static int detach_buf_packed(struct vring_virtqueue *vq, unsigned int head,
> > > > +			      void **ctx)
> > > > +{
> > > > +	struct vring_packed_desc *desc;
> > > > +	unsigned int i, j;
> > > > +
> > > > +	/* Clear data ptr. */
> > > > +	vq->desc_state[head].data = NULL;
> > > > +
> > > > +	i = head;
> > > > +
> > > > +	for (j = 0; j < vq->desc_state[head].num; j++) {
> > > > +		desc = &vq->vring_packed.desc[i];
> > > > +		vring_unmap_one_packed(vq, desc);
> > > > +		desc->flags = 0x0;
> > > Looks like this is unnecessary.
> > It's safer to zero it. If we don't zero it, after we
> > call virtqueue_detach_unused_buf_packed() which calls
> > this function, the desc is still available to the
> > device.
> 
> Well detach_unused_buf_packed() should be called after device is stopped,
> otherwise even if you try to clear, there will still be a window that device
> may use it.
> 
> > 
> > > > +		i++;
> > > > +		if (i >= vq->vring_packed.num)
> > > > +			i = 0;
> > > > +	}
> > [...]
> > > > +static unsigned virtqueue_enable_cb_prepare_packed(struct virtqueue *_vq)
> > > > +{
> > > > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > > > +	u16 last_used_idx, wrap_counter, off_wrap;
> > > > +
> > > > +	START_USE(vq);
> > > > +
> > > > +	last_used_idx = vq->last_used_idx;
> > > > +	wrap_counter = vq->wrap_counter;
> > > > +
> > > > +	if (last_used_idx > vq->next_avail_idx)
> > > > +		wrap_counter ^= 1;
> > > > +
> > > > +	off_wrap = last_used_idx | (wrap_counter << 15);
> > > > +
> > > > +	/* We optimistically turn back on interrupts, then check if there was
> > > > +	 * more to do. */
> > > > +	/* Depending on the VIRTIO_RING_F_EVENT_IDX feature, we need to
> > > > +	 * either clear the flags bit or point the event index at the next
> > > > +	 * entry. Always do both to keep code simple. */
> > > > +	if (vq->event_flags_shadow == VRING_EVENT_F_DISABLE) {
> > > > +		vq->event_flags_shadow = vq->event ? VRING_EVENT_F_DESC:
> > > > +						     VRING_EVENT_F_ENABLE;
> > > > +		vq->vring_packed.driver->flags = cpu_to_virtio16(_vq->vdev,
> > > > +							vq->event_flags_shadow);
> > > > +	}
> > > A smp_wmb() is missed here?
> > > 
> > > > +	vq->vring_packed.driver->off_wrap = cpu_to_virtio16(_vq->vdev, off_wrap);
> > > And according to the spec, it looks to me write a VRING_EVENT_F_ENABLE is
> > > sufficient here.
> > I didn't think much when implementing the event suppression
> > for packed ring previously. After I saw your comments, I found
> > something new. Indeed, unlike the split ring, for the packed
> > ring, spec doesn't say we must use VRING_EVENT_F_DESC when
> > EVENT_IDX is negotiated. So do you think below thought is
> > right or makes sense?
> > 
> > - For virtqueue_enable_cb_prepare(), we just need to enable
> >    the ring by setting flags to VRING_EVENT_F_ENABLE in any
> >    case.
> > 
> > - We will try to use VRING_EVENT_F_DESC (if EVENT_IDX is
> >    negotiated) only when we want to delay the interrupts
> >    virtqueue_enable_cb_delayed().
> 
> This looks good to me.

I suspect this will lead to extra interrupts if host is fast.
So I think for now we should always use VRING_EVENT_F_DESC
if EVENT_IDX is negotiated.

VRING_EVENT_F_DISABLE makes more sense to me.

> > 
> > > > +	END_USE(vq);
> > > > +	return last_used_idx;
> > > > +}
> > > > +
> > [...]
> > > > @@ -1157,14 +1852,18 @@ void vring_transport_features(struct virtio_device *vdev)
> > > >    	for (i = VIRTIO_TRANSPORT_F_START; i < VIRTIO_TRANSPORT_F_END; i++) {
> > > >    		switch (i) {
> > > > -		case VIRTIO_RING_F_INDIRECT_DESC:
> > > > +#if 0
> > > > +		case VIRTIO_RING_F_INDIRECT_DESC: // FIXME not tested yet.
> > > >    			break;
> > > > -		case VIRTIO_RING_F_EVENT_IDX:
> > > > +		case VIRTIO_RING_F_EVENT_IDX: // FIXME probably not work.
> > > >    			break;
> > > > +#endif
> > > It would be better if you can split EVENT_IDX and INDIRECT_DESC into
> > > separate patches too.
> > Sure. Will do it in the next version.
> > 
> > Thanks for the review!
> 
> Thanks.
> 
> > > Thanks
> > > 
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