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Date:   Fri, 16 Mar 2018 14:10:47 +0800
From:   Tiwei Bie <tiwei.bie@intel.com>
To:     Jason Wang <jasowang@redhat.com>
Cc:     mst@redhat.com, virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        wexu@redhat.com, jfreimann@redhat.com
Subject: Re: [PATCH RFC 2/2] virtio_ring: support packed ring
Message-ID: <20180316061047.o2xdyuqhak3mzjyw@debian>
References: <20180223111801.15240-1-tiwei.bie@intel.com>
 <20180223111801.15240-3-tiwei.bie@intel.com>
 <8f73267a-e20e-de64-d8e0-3fd608dbf368@redhat.com>
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On Fri, Mar 16, 2018 at 12:03:25PM +0800, Jason Wang wrote:
> On 2018年02月23日 19:18, Tiwei Bie wrote:
> > Signed-off-by: Tiwei Bie <tiwei.bie@intel.com>
> > ---
> >   drivers/virtio/virtio_ring.c | 699 +++++++++++++++++++++++++++++++++++++------
> >   include/linux/virtio_ring.h  |   8 +-
> >   2 files changed, 618 insertions(+), 89 deletions(-)
> > 
> > diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
> > index eb30f3e09a47..393778a2f809 100644
> > --- a/drivers/virtio/virtio_ring.c
> > +++ b/drivers/virtio/virtio_ring.c
> > @@ -58,14 +58,14 @@
> >   struct vring_desc_state {
> >   	void *data;			/* Data for callback. */
> > -	struct vring_desc *indir_desc;	/* Indirect descriptor, if any. */
> > +	void *indir_desc;		/* Indirect descriptor, if any. */
> > +	int num;			/* Descriptor list length. */
> >   };
> >   struct vring_virtqueue {
> >   	struct virtqueue vq;
> > -	/* Actual memory layout for this queue */
> > -	struct vring vring;
> > +	bool packed;
> >   	/* Can we use weak barriers? */
> >   	bool weak_barriers;
> > @@ -87,11 +87,28 @@ struct vring_virtqueue {
> >   	/* Last used index we've seen. */
> >   	u16 last_used_idx;
> > -	/* Last written value to avail->flags */
> > -	u16 avail_flags_shadow;
> > -
> > -	/* Last written value to avail->idx in guest byte order */
> > -	u16 avail_idx_shadow;
> > +	union {
> > +		/* Available for split ring */
> > +		struct {
> > +			/* Actual memory layout for this queue */
> > +			struct vring vring;
> > +
> > +			/* Last written value to avail->flags */
> > +			u16 avail_flags_shadow;
> > +
> > +			/* Last written value to avail->idx in
> > +			 * guest byte order */
> > +			u16 avail_idx_shadow;
> > +		};
> > +
> > +		/* Available for packed ring */
> > +		struct {
> > +			/* Actual memory layout for this queue */
> > +			struct vring_packed vring_packed;
> > +			u8 wrap_counter : 1;
> > +			bool chaining;
> > +		};
> > +	};
> >   	/* How to notify other side. FIXME: commonalize hcalls! */
> >   	bool (*notify)(struct virtqueue *vq);
> > @@ -201,26 +218,37 @@ static dma_addr_t vring_map_single(const struct vring_virtqueue *vq,
> >   			      cpu_addr, size, direction);
> >   }
> > -static void vring_unmap_one(const struct vring_virtqueue *vq,
> > -			    struct vring_desc *desc)
> > +static void vring_unmap_one(const struct vring_virtqueue *vq, void *_desc)
> >   {
> 
> Let's split the helpers to packed/split version like other helpers?
> (Consider the caller has already known the type of vq).

Okay.

> 
> > +	u64 addr;
> > +	u32 len;
> >   	u16 flags;
> >   	if (!vring_use_dma_api(vq->vq.vdev))
> >   		return;
> > -	flags = virtio16_to_cpu(vq->vq.vdev, desc->flags);
> > +	if (vq->packed) {
> > +		struct vring_packed_desc *desc = _desc;
> > +
> > +		addr = virtio64_to_cpu(vq->vq.vdev, desc->addr);
> > +		len = virtio32_to_cpu(vq->vq.vdev, desc->len);
> > +		flags = virtio16_to_cpu(vq->vq.vdev, desc->flags);
> > +	} else {
> > +		struct vring_desc *desc = _desc;
> > +
> > +		addr = virtio64_to_cpu(vq->vq.vdev, desc->addr);
> > +		len = virtio32_to_cpu(vq->vq.vdev, desc->len);
> > +		flags = virtio16_to_cpu(vq->vq.vdev, desc->flags);
> > +	}
> >   	if (flags & VRING_DESC_F_INDIRECT) {
> >   		dma_unmap_single(vring_dma_dev(vq),
> > -				 virtio64_to_cpu(vq->vq.vdev, desc->addr),
> > -				 virtio32_to_cpu(vq->vq.vdev, desc->len),
> > +				 addr, len,
> >   				 (flags & VRING_DESC_F_WRITE) ?
> >   				 DMA_FROM_DEVICE : DMA_TO_DEVICE);
> >   	} else {
> >   		dma_unmap_page(vring_dma_dev(vq),
> > -			       virtio64_to_cpu(vq->vq.vdev, desc->addr),
> > -			       virtio32_to_cpu(vq->vq.vdev, desc->len),
> > +			       addr, len,
> >   			       (flags & VRING_DESC_F_WRITE) ?
> >   			       DMA_FROM_DEVICE : DMA_TO_DEVICE);
> >   	}
> > @@ -235,8 +263,9 @@ static int vring_mapping_error(const struct vring_virtqueue *vq,
> >   	return dma_mapping_error(vring_dma_dev(vq), addr);
> >   }
> > -static struct vring_desc *alloc_indirect(struct virtqueue *_vq,
> > -					 unsigned int total_sg, gfp_t gfp)
> > +static struct vring_desc *alloc_indirect_split(struct virtqueue *_vq,
> > +					       unsigned int total_sg,
> > +					       gfp_t gfp)
> >   {
> >   	struct vring_desc *desc;
> >   	unsigned int i;
> > @@ -257,14 +286,32 @@ static struct vring_desc *alloc_indirect(struct virtqueue *_vq,
> >   	return desc;
> >   }
> > -static inline int virtqueue_add(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)
> > +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 inline int virtqueue_add_split(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 scatterlist *sg;
> > @@ -303,7 +350,7 @@ static inline int virtqueue_add(struct virtqueue *_vq,
> >   	/* 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)
> > -		desc = alloc_indirect(_vq, total_sg, gfp);
> > +		desc = alloc_indirect_split(_vq, total_sg, gfp);
> >   	else {
> >   		desc = NULL;
> >   		WARN_ON_ONCE(total_sg > vq->vring.num && !vq->indirect);
> > @@ -437,6 +484,243 @@ static inline int virtqueue_add(struct virtqueue *_vq,
> >   	return -EIO;
> >   }
> > +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;
> > +	}
> > +
> > +	if (total_sg > 1 && !vq->chaining && !vq->indirect) {
> > +		END_USE(vq);
> > +		return -ENOTSUPP;
> > +	}
> > +
> > +#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->free_head;
> > +	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)
> > +		desc = alloc_indirect_packed(_vq, total_sg, gfp);
> > +	else {
> > +		desc = NULL;
> > +		WARN_ON_ONCE(total_sg > vq->vring.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 (total_sg > 1 && !vq->chaining) {
> > +			END_USE(vq);
> > +			return -ENOTSUPP;
> > +		}
> > +	}
> > +
> > +	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; n++) {
> > +		for (sg = sgs[n]; sg; sg = sg_next(sg)) {
> > +			dma_addr_t addr = vring_map_one_sg(vq, sg, DMA_TO_DEVICE);
> > +			if (vring_mapping_error(vq, addr))
> > +				goto unmap_release;
> > +
> > +			flags = cpu_to_virtio16(_vq->vdev, VRING_DESC_F_NEXT |
> > +					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);
> 
> If it's a part of chain, we only need to do this for last buffer I think.

I'm not sure I've got your point about the "last buffer".
But, yes, id just needs to be set for the last desc.

> 
> > +			prev = i;
> > +			i++;
> 
> It looks to me prev is always i - 1?

No. prev will be (vq->vring_packed.num - 1) when i becomes 0.

> 
> > +			if (!indirect && i >= vq->vring_packed.num) {
> > +				i = 0;
> > +				vq->wrap_counter ^= 1;
> > +			}
> > +		}
> > +	}
> > +	for (; 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, DMA_FROM_DEVICE);
> > +			if (vring_mapping_error(vq, addr))
> > +				goto unmap_release;
> > +
> > +			flags = cpu_to_virtio16(_vq->vdev, VRING_DESC_F_NEXT |
> > +					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);
> > +			prev = i;
> > +			i++;
> > +			if (!indirect && i >= vq->vring_packed.num) {
> > +				i = 0;
> > +				vq->wrap_counter ^= 1;
> > +			}
> > +		}
> > +	}
> > +	/* Last one doesn't continue. */
> > +	if (!indirect && (head + 1) % vq->vring_packed.num == i)
> > +		head_flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
> 
> I can't get the why we need this here.

If only one desc is used, we will need to clear the
VRING_DESC_F_NEXT flag from the head_flags.

> 
> > +	else
> > +		desc[prev].flags &= cpu_to_virtio16(_vq->vdev, ~VRING_DESC_F_NEXT);
> > +
> > +	if (indirect) {
> > +		/* FIXME: to be implemented */
> > +
> > +		/* Now that the indirect table is filled in, map it. */
> > +		dma_addr_t addr = vring_map_single(
> > +			vq, desc, total_sg * sizeof(struct vring_packed_desc),
> > +			DMA_TO_DEVICE);
> > +		if (vring_mapping_error(vq, addr))
> > +			goto unmap_release;
> > +
> > +		head_flags = cpu_to_virtio16(_vq->vdev, VRING_DESC_F_INDIRECT |
> > +					     VRING_DESC_F_AVAIL(wrap_counter) |
> > +					     VRING_DESC_F_USED(!wrap_counter));
> > +		vq->vring_packed.desc[head].addr = cpu_to_virtio64(_vq->vdev, addr);
> > +		vq->vring_packed.desc[head].len = cpu_to_virtio32(_vq->vdev,
> > +				total_sg * sizeof(struct vring_packed_desc));
> > +		vq->vring_packed.desc[head].id = cpu_to_virtio32(_vq->vdev, head);
> > +	}
> > +
> > +	/* We're using some buffers from the free list. */
> > +	vq->vq.num_free -= descs_used;
> > +
> > +	/* Update free pointer */
> > +	if (indirect) {
> > +		n = head + 1;
> > +		if (n >= vq->vring_packed.num) {
> > +			n = 0;
> > +			vq->wrap_counter ^= 1;
> > +		}
> > +		vq->free_head = n;
> 
> detach_buf_packed() does not even touch free_head here, so need to explain
> its meaning for packed ring.

Above code is for indirect support which isn't really
implemented in this patch yet.

For your question, free_head stores the index of the
next avail desc. I'll add a comment for it or move it
to union and give it a better name in next version.

> 
> > +	} else
> > +		vq->free_head = i;
> 
> ID is only valid in the last descriptor in the list, so head + 1 should be
> ok too?

I don't really get your point. The vq->free_head stores
the index of the next avail desc.

> 
> > +
> > +	/* Store token and indirect buffer state. */
> > +	vq->desc_state[head].num = descs_used;
> > +	vq->desc_state[head].data = data;
> > +	if (indirect)
> > +		vq->desc_state[head].indir_desc = desc;
> > +	else
> > +		vq->desc_state[head].indir_desc = ctx;
> > +
> > +	virtio_wmb(vq->weak_barriers);
> 
> Let's add a comment to explain the barrier here.

Okay.

> 
> > +	vq->vring_packed.desc[head].flags = head_flags;
> > +	vq->num_added++;
> > +
> > +	pr_debug("Added buffer head %i to %p\n", head, vq);
> > +	END_USE(vq);
> > +
> > +	return 0;
> > +
> > +unmap_release:
> > +	err_idx = i;
> > +	i = head;
> > +
> > +	for (n = 0; n < total_sg; n++) {
> > +		if (i == err_idx)
> > +			break;
> > +		vring_unmap_one(vq, &desc[i]);
> > +		i++;
> > +		if (!indirect && i >= vq->vring_packed.num)
> > +			i = 0;
> > +	}
> > +
> > +	vq->wrap_counter = wrap_counter;
> > +
> > +	if (indirect)
> > +		kfree(desc);
> > +
> > +	END_USE(vq);
> > +	return -EIO;
> > +}
> > +
> > +static inline int virtqueue_add(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);
> > +
> > +	return vq->packed ? virtqueue_add_packed(_vq, sgs, total_sg, out_sgs,
> > +						 in_sgs, data, ctx, gfp) :
> > +			    virtqueue_add_split(_vq, sgs, total_sg, out_sgs,
> > +						in_sgs, data, ctx, gfp);
> > +}
> > +
> >   /**
> >    * virtqueue_add_sgs - expose buffers to other end
> >    * @vq: the struct virtqueue we're talking about.
> > @@ -561,6 +845,12 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
> >   	 * event. */
> >   	virtio_mb(vq->weak_barriers);
> > +	if (vq->packed) {
> > +		/* FIXME: to be implemented */
> > +		needs_kick = true;
> > +		goto out;
> > +	}
> > +
> >   	old = vq->avail_idx_shadow - vq->num_added;
> >   	new = vq->avail_idx_shadow;
> >   	vq->num_added = 0;
> > @@ -579,6 +869,8 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq)
> >   	} else {
> >   		needs_kick = !(vq->vring.used->flags & cpu_to_virtio16(_vq->vdev, VRING_USED_F_NO_NOTIFY));
> >   	}
> > +
> > +out:
> >   	END_USE(vq);
> >   	return needs_kick;
> >   }
> > @@ -628,8 +920,8 @@ bool virtqueue_kick(struct virtqueue *vq)
> >   }
> >   EXPORT_SYMBOL_GPL(virtqueue_kick);
> > -static void detach_buf(struct vring_virtqueue *vq, unsigned int head,
> > -		       void **ctx)
> > +static void detach_buf_split(struct vring_virtqueue *vq, unsigned int head,
> > +			     void **ctx)
> >   {
> >   	unsigned int i, j;
> >   	__virtio16 nextflag = cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_NEXT);
> > @@ -677,29 +969,81 @@ static void detach_buf(struct vring_virtqueue *vq, unsigned int head,
> >   	}
> >   }
> > -static inline bool more_used(const struct vring_virtqueue *vq)
> > +static void 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(vq, desc);
> > +		i++;
> > +		if (i >= vq->vring_packed.num)
> > +			i = 0;
> > +	}
> > +
> > +	vq->vq.num_free += vq->desc_state[head].num;
> 
> It looks to me vq->free_head grows always, how can we make sure it does not
> exceeds vq.num here?

The vq->free_head stores the index of the next avail
desc. You can find it wraps together with vq->wrap_counter
in virtqueue_add_packed().

> 
> > +
> > +	if (vq->indirect) {
> > +		u32 len;
> > +
> > +		desc = vq->desc_state[head].indir_desc;
> > +		/* Free the indirect table, if any, now that it's unmapped. */
> > +		if (!desc)
> > +			return;
> > +
> > +		len = virtio32_to_cpu(vq->vq.vdev,
> > +				      vq->vring_packed.desc[head].len);
> > +
> > +		BUG_ON(!(vq->vring_packed.desc[head].flags &
> > +			 cpu_to_virtio16(vq->vq.vdev, VRING_DESC_F_INDIRECT)));
> > +		BUG_ON(len == 0 || len % sizeof(struct vring_packed_desc));
> > +
> > +		for (j = 0; j < len / sizeof(struct vring_packed_desc); j++)
> > +			vring_unmap_one(vq, &desc[j]);
> > +
> > +		kfree(desc);
> > +		vq->desc_state[head].indir_desc = NULL;
> > +	} else if (ctx) {
> > +		*ctx = vq->desc_state[head].indir_desc;
> > +	}
> > +}
> > +
> > +static inline bool more_used_split(const struct vring_virtqueue *vq)
> >   {
> >   	return vq->last_used_idx != virtio16_to_cpu(vq->vq.vdev, vq->vring.used->idx);
> >   }
> > -/**
> > - * virtqueue_get_buf - get the next used buffer
> > - * @vq: the struct virtqueue we're talking about.
> > - * @len: the length written into the buffer
> > - *
> > - * If the device wrote data into the buffer, @len will be set to the
> > - * amount written.  This means you don't need to clear the buffer
> > - * beforehand to ensure there's no data leakage in the case of short
> > - * writes.
> > - *
> > - * Caller must ensure we don't call this with other virtqueue
> > - * operations at the same time (except where noted).
> > - *
> > - * Returns NULL if there are no used buffers, or the "data" token
> > - * handed to virtqueue_add_*().
> > - */
> > -void *virtqueue_get_buf_ctx(struct virtqueue *_vq, unsigned int *len,
> > -			    void **ctx)
> > +static inline bool more_used_packed(const struct vring_virtqueue *vq)
> > +{
> > +	u16 last_used, flags;
> > +	bool avail, used;
> > +
> > +	if (vq->vq.num_free == vq->vring.num)
> > +		return false;
> > +
> > +	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;
> > +}
> > +
> > +static inline bool more_used(const struct vring_virtqueue *vq)
> > +{
> > +	return vq->packed ? more_used_packed(vq) : more_used_split(vq);
> > +}
> > +
> > +void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq, unsigned int *len,
> > +				  void **ctx)
> >   {
> >   	struct vring_virtqueue *vq = to_vvq(_vq);
> >   	void *ret;
> > @@ -735,9 +1079,9 @@ void *virtqueue_get_buf_ctx(struct virtqueue *_vq, unsigned int *len,
> >   		return NULL;
> >   	}
> > -	/* detach_buf clears data, so grab it now. */
> > +	/* detach_buf_split clears data, so grab it now. */
> >   	ret = vq->desc_state[i].data;
> > -	detach_buf(vq, i, ctx);
> > +	detach_buf_split(vq, i, ctx);
> >   	vq->last_used_idx++;
> >   	/* If we expect an interrupt for the next entry, tell host
> >   	 * by writing event index and flush out the write before
> > @@ -754,6 +1098,87 @@ void *virtqueue_get_buf_ctx(struct virtqueue *_vq, unsigned int *len,
> >   	END_USE(vq);
> >   	return ret;
> >   }
> > +
> > +void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq, unsigned int *len,
> > +				   void **ctx)
> > +{
> > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > +	void *ret;
> > +	unsigned int i;
> > +	u16 last_used;
> > +
> > +	START_USE(vq);
> > +
> > +	if (unlikely(vq->broken)) {
> > +		END_USE(vq);
> > +		return NULL;
> > +	}
> > +
> > +	if (!more_used(vq)) {
> > +		pr_debug("No more buffers in queue\n");
> > +		END_USE(vq);
> > +		return NULL;
> > +	}
> > +
> > +	/* Only get used array entries after they have been exposed by host. */
> > +	virtio_rmb(vq->weak_barriers);
> > +
> > +	last_used = vq->last_used_idx;
> > +
> > +	i = virtio32_to_cpu(_vq->vdev, vq->vring_packed.desc[last_used].id);
> > +	*len = virtio32_to_cpu(_vq->vdev, vq->vring_packed.desc[last_used].len);
> > +
> > +	if (unlikely(i >= vq->vring_packed.num)) {
> > +		BAD_RING(vq, "id %u out of range\n", i);
> > +		return NULL;
> > +	}
> > +	if (unlikely(!vq->desc_state[i].data)) {
> > +		BAD_RING(vq, "id %u is not a head!\n", i);
> > +		return NULL;
> > +	}
> > +
> > +	/* detach_buf_packed clears data, so grab it now. */
> > +	ret = vq->desc_state[i].data;
> > +	detach_buf_packed(vq, i, ctx);
> > +
> > +	vq->last_used_idx += vq->desc_state[i].num;
> > +	if (vq->last_used_idx >= vq->vring_packed.num)
> > +		vq->last_used_idx %= vq->vring_packed.num;
> 
> '-=' should be sufficient here?

Good suggestion. I think so.

> 
> > +
> > +	// FIXME: implement the desc event support
> > +
> > +#ifdef DEBUG
> > +	vq->last_add_time_valid = false;
> > +#endif
> > +
> > +	END_USE(vq);
> > +	return ret;
> > +}
> > +
> > +/**
> > + * virtqueue_get_buf - get the next used buffer
> > + * @vq: the struct virtqueue we're talking about.
> > + * @len: the length written into the buffer
> > + *
> > + * If the device wrote data into the buffer, @len will be set to the
> > + * amount written.  This means you don't need to clear the buffer
> > + * beforehand to ensure there's no data leakage in the case of short
> > + * writes.
> > + *
> > + * Caller must ensure we don't call this with other virtqueue
> > + * operations at the same time (except where noted).
> > + *
> > + * Returns NULL if there are no used buffers, or the "data" token
> > + * handed to virtqueue_add_*().
> > + */
> > +void *virtqueue_get_buf_ctx(struct virtqueue *_vq, unsigned int *len,
> > +			    void **ctx)
> > +{
> > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > +
> > +	return vq->packed ? virtqueue_get_buf_ctx_packed(_vq, len, ctx) :
> > +			    virtqueue_get_buf_ctx_split(_vq, len, ctx);
> > +}
> >   EXPORT_SYMBOL_GPL(virtqueue_get_buf_ctx);
> >   void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
> > @@ -761,6 +1186,24 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len)
> >   	return virtqueue_get_buf_ctx(_vq, len, NULL);
> >   }
> >   EXPORT_SYMBOL_GPL(virtqueue_get_buf);
> > +
> > +static void virtqueue_disable_cb_split(struct virtqueue *_vq)
> > +{
> > +	struct vring_virtqueue *vq = to_vvq(_vq);
> > +
> > +	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
> > +		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> > +		if (!vq->event)
> > +			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev,
> > +							vq->avail_flags_shadow);
> > +	}
> > +}
> > +
> > +static void virtqueue_disable_cb_packed(struct virtqueue *_vq)
> > +{
> > +	// FIXME: to be implemented
> > +}
> > +
> >   /**
> >    * virtqueue_disable_cb - disable callbacks
> >    * @vq: the struct virtqueue we're talking about.
> > @@ -774,12 +1217,10 @@ void virtqueue_disable_cb(struct virtqueue *_vq)
> >   {
> >   	struct vring_virtqueue *vq = to_vvq(_vq);
> > -	if (!(vq->avail_flags_shadow & VRING_AVAIL_F_NO_INTERRUPT)) {
> > -		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> > -		if (!vq->event)
> > -			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow);
> > -	}
> > -
> > +	if (vq->packed)
> > +		virtqueue_disable_cb_packed(_vq);
> > +	else
> > +		virtqueue_disable_cb_split(_vq);
> >   }
> >   EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
> > @@ -802,6 +1243,12 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
> >   	START_USE(vq);
> > +	if (vq->packed) {
> > +		// FIXME: to be implemented
> > +		last_used_idx = vq->last_used_idx;
> > +		goto out;
> > +	}
> > +
> >   	/* 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
> > @@ -813,6 +1260,7 @@ unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
> >   			vq->vring.avail->flags = cpu_to_virtio16(_vq->vdev, vq->avail_flags_shadow);
> >   	}
> >   	vring_used_event(&vq->vring) = cpu_to_virtio16(_vq->vdev, last_used_idx = vq->last_used_idx);
> > +out:
> >   	END_USE(vq);
> >   	return last_used_idx;
> >   }
> > @@ -832,6 +1280,12 @@ bool virtqueue_poll(struct virtqueue *_vq, unsigned last_used_idx)
> >   	struct vring_virtqueue *vq = to_vvq(_vq);
> >   	virtio_mb(vq->weak_barriers);
> > +	if (vq->packed) {
> > +		u16 flags = virtio16_to_cpu(vq->vq.vdev,
> > +				vq->vring_packed.desc[last_used_idx].flags);
> > +		return !(flags & VRING_DESC_F_AVAIL(1)) ==
> > +		       !(flags & VRING_DESC_F_USED(1));
> > +	}
> >   	return (u16)last_used_idx != virtio16_to_cpu(_vq->vdev, vq->vring.used->idx);
> >   }
> >   EXPORT_SYMBOL_GPL(virtqueue_poll);
> > @@ -874,6 +1328,11 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
> >   	START_USE(vq);
> > +	if (vq->packed) {
> > +		// FIXME: to be implemented
> > +		goto out;
> > +	}
> > +
> >   	/* We optimistically turn back on interrupts, then check if there was
> >   	 * more to do. */
> >   	/* Depending on the VIRTIO_RING_F_USED_EVENT_IDX feature, we need to
> > @@ -896,6 +1355,7 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq)
> >   		return false;
> >   	}
> > +out:
> >   	END_USE(vq);
> >   	return true;
> >   }
> > @@ -922,14 +1382,20 @@ void *virtqueue_detach_unused_buf(struct virtqueue *_vq)
> >   			continue;
> >   		/* detach_buf clears data, so grab it now. */
> >   		buf = vq->desc_state[i].data;
> > -		detach_buf(vq, i, NULL);
> > -		vq->avail_idx_shadow--;
> > -		vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev, vq->avail_idx_shadow);
> > +		if (vq->packed)
> > +			detach_buf_packed(vq, i, NULL);
> > +		else {
> > +			detach_buf_split(vq, i, NULL);
> > +			vq->avail_idx_shadow--;
> > +			vq->vring.avail->idx = cpu_to_virtio16(_vq->vdev,
> > +							vq->avail_idx_shadow);
> > +		}
> >   		END_USE(vq);
> >   		return buf;
> >   	}
> >   	/* That should have freed everything. */
> > -	BUG_ON(vq->vq.num_free != vq->vring.num);
> > +	BUG_ON(vq->vq.num_free != (vq->packed ? vq->vring_packed.num :
> > +						vq->vring.num));
> >   	END_USE(vq);
> >   	return NULL;
> > @@ -957,7 +1423,8 @@ irqreturn_t vring_interrupt(int irq, void *_vq)
> >   EXPORT_SYMBOL_GPL(vring_interrupt);
> >   struct virtqueue *__vring_new_virtqueue(unsigned int index,
> > -					struct vring vring,
> > +					union vring_union vring,
> > +					bool packed,
> >   					struct virtio_device *vdev,
> >   					bool weak_barriers,
> >   					bool context,
> > @@ -965,19 +1432,20 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
> >   					void (*callback)(struct virtqueue *),
> >   					const char *name)
> >   {
> > -	unsigned int i;
> > +	unsigned int num, i;
> >   	struct vring_virtqueue *vq;
> > -	vq = kmalloc(sizeof(*vq) + vring.num * sizeof(struct vring_desc_state),
> > +	num = packed ? vring.vring_packed.num : vring.vring_split.num;
> > +
> > +	vq = kmalloc(sizeof(*vq) + num * sizeof(struct vring_desc_state),
> >   		     GFP_KERNEL);
> >   	if (!vq)
> >   		return NULL;
> > -	vq->vring = vring;
> >   	vq->vq.callback = callback;
> >   	vq->vq.vdev = vdev;
> >   	vq->vq.name = name;
> > -	vq->vq.num_free = vring.num;
> > +	vq->vq.num_free = num;
> >   	vq->vq.index = index;
> >   	vq->we_own_ring = false;
> >   	vq->queue_dma_addr = 0;
> > @@ -986,9 +1454,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
> >   	vq->weak_barriers = weak_barriers;
> >   	vq->broken = false;
> >   	vq->last_used_idx = 0;
> > -	vq->avail_flags_shadow = 0;
> > -	vq->avail_idx_shadow = 0;
> >   	vq->num_added = 0;
> > +	vq->packed = packed;
> >   	list_add_tail(&vq->vq.list, &vdev->vqs);
> >   #ifdef DEBUG
> >   	vq->in_use = false;
> > @@ -999,18 +1466,41 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
> >   		!context;
> >   	vq->event = virtio_has_feature(vdev, VIRTIO_RING_F_EVENT_IDX);
> > +	if (vq->packed) {
> > +		vq->vring_packed = vring.vring_packed;
> > +		vq->free_head = 0;
> > +		vq->wrap_counter = 1;
> > +
> > +#if 0
> > +		vq->chaining = virtio_has_feature(vdev,
> > +						  VIRTIO_RING_F_LIST_DESC);
> > +#else
> > +		vq->chaining = true;
> 
> Looks like in V10 there's no F_LIST_DESC.

Yes. I kept this in this patch just because the
desc chaining is optional in the old spec draft
when sending out this patch set. I'll remove it
in next version.

> 
> > +#endif
> > +	} else {
> > +		vq->vring = vring.vring_split;
> > +		vq->avail_flags_shadow = 0;
> > +		vq->avail_idx_shadow = 0;
> > +
> > +		/* Put everything in free lists. */
> > +		vq->free_head = 0;
> > +		for (i = 0; i < num-1; i++)
> > +			vq->vring.desc[i].next = cpu_to_virtio16(vdev, i + 1);
> > +	}
> > +
> >   	/* No callback?  Tell other side not to bother us. */
> >   	if (!callback) {
> > -		vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> > -		if (!vq->event)
> > -			vq->vring.avail->flags = cpu_to_virtio16(vdev, vq->avail_flags_shadow);
> > +		if (packed) {
> > +			// FIXME: to be implemented
> > +		} else {
> > +			vq->avail_flags_shadow |= VRING_AVAIL_F_NO_INTERRUPT;
> > +			if (!vq->event)
> > +				vq->vring.avail->flags = cpu_to_virtio16(vdev,
> > +						vq->avail_flags_shadow);
> > +		}
> >   	}
> > -	/* Put everything in free lists. */
> > -	vq->free_head = 0;
> > -	for (i = 0; i < vring.num-1; i++)
> > -		vq->vring.desc[i].next = cpu_to_virtio16(vdev, i + 1);
> > -	memset(vq->desc_state, 0, vring.num * sizeof(struct vring_desc_state));
> > +	memset(vq->desc_state, 0, num * sizeof(struct vring_desc_state));
> >   	return &vq->vq;
> >   }
> > @@ -1058,6 +1548,14 @@ static void vring_free_queue(struct virtio_device *vdev, size_t size,
> >   	}
> >   }
> > +static inline int
> > +__vring_size(unsigned int num, unsigned long align, bool packed)
> > +{
> > +	if (packed)
> > +		return vring_packed_size(num, align);
> > +	return vring_size(num, align);
> > +}
> > +
> >   struct virtqueue *vring_create_virtqueue(
> >   	unsigned int index,
> >   	unsigned int num,
> > @@ -1074,7 +1572,8 @@ struct virtqueue *vring_create_virtqueue(
> >   	void *queue = NULL;
> >   	dma_addr_t dma_addr;
> >   	size_t queue_size_in_bytes;
> > -	struct vring vring;
> > +	union vring_union vring;
> > +	bool packed;
> >   	/* We assume num is a power of 2. */
> >   	if (num & (num - 1)) {
> > @@ -1082,9 +1581,13 @@ struct virtqueue *vring_create_virtqueue(
> >   		return NULL;
> >   	}
> > +	packed = virtio_has_feature(vdev, VIRTIO_F_RING_PACKED);
> > +
> >   	/* TODO: allocate each queue chunk individually */
> > -	for (; num && vring_size(num, vring_align) > PAGE_SIZE; num /= 2) {
> > -		queue = vring_alloc_queue(vdev, vring_size(num, vring_align),
> > +	for (; num && __vring_size(num, vring_align, packed) > PAGE_SIZE;
> > +			num /= 2) {
> > +		queue = vring_alloc_queue(vdev, __vring_size(num, vring_align,
> > +							     packed),
> >   					  &dma_addr,
> >   					  GFP_KERNEL|__GFP_NOWARN|__GFP_ZERO);
> >   		if (queue)
> > @@ -1096,17 +1599,21 @@ struct virtqueue *vring_create_virtqueue(
> >   	if (!queue) {
> >   		/* Try to get a single page. You are my only hope! */
> > -		queue = vring_alloc_queue(vdev, vring_size(num, vring_align),
> > +		queue = vring_alloc_queue(vdev, __vring_size(num, vring_align,
> > +							     packed),
> >   					  &dma_addr, GFP_KERNEL|__GFP_ZERO);
> >   	}
> >   	if (!queue)
> >   		return NULL;
> > -	queue_size_in_bytes = vring_size(num, vring_align);
> > -	vring_init(&vring, num, queue, vring_align);
> > +	queue_size_in_bytes = __vring_size(num, vring_align, packed);
> > +	if (packed)
> > +		vring_packed_init(&vring.vring_packed, num, queue, vring_align);
> > +	else
> > +		vring_init(&vring.vring_split, num, queue, vring_align);
> 
> Let's rename vring_init to vring_init_split() like other helpers?

The vring_init() is a public API in include/uapi/linux/virtio_ring.h.
I don't think we can rename it.

> 
> > -	vq = __vring_new_virtqueue(index, vring, vdev, weak_barriers, context,
> > -				   notify, callback, name);
> > +	vq = __vring_new_virtqueue(index, vring, packed, vdev, weak_barriers,
> > +				   context, notify, callback, name);
> >   	if (!vq) {
> >   		vring_free_queue(vdev, queue_size_in_bytes, queue,
> >   				 dma_addr);
> > @@ -1132,10 +1639,17 @@ struct virtqueue *vring_new_virtqueue(unsigned int index,
> >   				      void (*callback)(struct virtqueue *vq),
> >   				      const char *name)
> >   {
> > -	struct vring vring;
> > -	vring_init(&vring, num, pages, vring_align);
> > -	return __vring_new_virtqueue(index, vring, vdev, weak_barriers, context,
> > -				     notify, callback, name);
> > +	union vring_union vring;
> > +	bool packed;
> > +
> > +	packed = virtio_has_feature(vdev, VIRTIO_F_RING_PACKED);
> > +	if (packed)
> > +		vring_packed_init(&vring.vring_packed, num, pages, vring_align);
> > +	else
> > +		vring_init(&vring.vring_split, num, pages, vring_align);
> > +
> > +	return __vring_new_virtqueue(index, vring, packed, vdev, weak_barriers,
> > +				     context, notify, callback, name);
> >   }
> >   EXPORT_SYMBOL_GPL(vring_new_virtqueue);
> > @@ -1145,7 +1659,9 @@ void vring_del_virtqueue(struct virtqueue *_vq)
> >   	if (vq->we_own_ring) {
> >   		vring_free_queue(vq->vq.vdev, vq->queue_size_in_bytes,
> > -				 vq->vring.desc, vq->queue_dma_addr);
> > +				 vq->packed ? (void *)vq->vring_packed.desc :
> > +					      (void *)vq->vring.desc,
> > +				 vq->queue_dma_addr);
> >   	}
> >   	list_del(&_vq->list);
> >   	kfree(vq);
> > @@ -1159,14 +1675,18 @@ void vring_transport_features(struct virtio_device *vdev)
> >   	for (i = VIRTIO_TRANSPORT_F_START; i < VIRTIO_TRANSPORT_F_END; i++) {
> >   		switch (i) {
> > +#if 0 // FIXME: to be implemented
> >   		case VIRTIO_RING_F_INDIRECT_DESC:
> >   			break;
> > +#endif
> >   		case VIRTIO_RING_F_EVENT_IDX:
> >   			break;
> >   		case VIRTIO_F_VERSION_1:
> >   			break;
> >   		case VIRTIO_F_IOMMU_PLATFORM:
> >   			break;
> > +		case VIRTIO_F_RING_PACKED:
> > +			break;
> >   		default:
> >   			/* We don't understand this bit. */
> >   			__virtio_clear_bit(vdev, i);
> > @@ -1187,7 +1707,7 @@ unsigned int virtqueue_get_vring_size(struct virtqueue *_vq)
> >   	struct vring_virtqueue *vq = to_vvq(_vq);
> > -	return vq->vring.num;
> > +	return vq->packed ? vq->vring_packed.num : vq->vring.num;
> >   }
> >   EXPORT_SYMBOL_GPL(virtqueue_get_vring_size);
> > @@ -1224,6 +1744,7 @@ dma_addr_t virtqueue_get_desc_addr(struct virtqueue *_vq)
> >   }
> >   EXPORT_SYMBOL_GPL(virtqueue_get_desc_addr);
> > +/* Only available for split ring */
> 
> Interesting, I think we need this for correctly configure pci. e.g in
> setup_vq()?

Yes. The setup_vq() should be updated. But it requires
QEMU change, so I just kept it as is in this RFC patch.

> 
> >   dma_addr_t virtqueue_get_avail_addr(struct virtqueue *_vq)
> >   {
> >   	struct vring_virtqueue *vq = to_vvq(_vq);
> > @@ -1235,6 +1756,7 @@ dma_addr_t virtqueue_get_avail_addr(struct virtqueue *_vq)
> >   }
> >   EXPORT_SYMBOL_GPL(virtqueue_get_avail_addr);
> > +/* Only available for split ring */
> >   dma_addr_t virtqueue_get_used_addr(struct virtqueue *_vq)
> 
> Maybe it's better to rename this to get_device_addr().

It's a kernel API which has been exported by EXPORT_SYMBOL_GPL(),
I'm not sure whether it's a good idea to rename it.

Best regards,
Tiwei Bie

> 
> Thanks
> 
> >   {
> >   	struct vring_virtqueue *vq = to_vvq(_vq);
> > @@ -1246,6 +1768,7 @@ dma_addr_t virtqueue_get_used_addr(struct virtqueue *_vq)
> >   }
> >   EXPORT_SYMBOL_GPL(virtqueue_get_used_addr);
> > +/* Only available for split ring */
> >   const struct vring *virtqueue_get_vring(struct virtqueue *vq)
> >   {
> >   	return &to_vvq(vq)->vring;
> > diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h
> > index bbf32524ab27..a0075894ad16 100644
> > --- a/include/linux/virtio_ring.h
> > +++ b/include/linux/virtio_ring.h
> > @@ -60,6 +60,11 @@ static inline void virtio_store_mb(bool weak_barriers,
> >   struct virtio_device;
> >   struct virtqueue;
> > +union vring_union {
> > +	struct vring vring_split;
> > +	struct vring_packed vring_packed;
> > +};
> > +
> >   /*
> >    * Creates a virtqueue and allocates the descriptor ring.  If
> >    * may_reduce_num is set, then this may allocate a smaller ring than
> > @@ -79,7 +84,8 @@ struct virtqueue *vring_create_virtqueue(unsigned int index,
> >   /* Creates a virtqueue with a custom layout. */
> >   struct virtqueue *__vring_new_virtqueue(unsigned int index,
> > -					struct vring vring,
> > +					union vring_union vring,
> > +					bool packed,
> >   					struct virtio_device *vdev,
> >   					bool weak_barriers,
> >   					bool ctx,
>