Subject: Re: [PATCH RFC 01/13] vhost: option to fetch descriptors through an independent struct

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 3 Jun 2020 05:48:11 -0400

On Wed, Jun 03, 2020 at 03:13:56PM +0800, Jason Wang wrote:
> 
> On 2020/6/2 下午9:05, Michael S. Tsirkin wrote:
> > The idea is to support multiple ring formats by converting
> > to a format-independent array of descriptors.
> > 
> > This costs extra cycles, but we gain in ability
> > to fetch a batch of descriptors in one go, which
> > is good for code cache locality.
> > 
> > When used, this causes a minor performance degradation,
> > it's been kept as simple as possible for ease of review.
> > A follow-up patch gets us back the performance by adding batching.
> > 
> > To simplify benchmarking, I kept the old code around so one can switch
> > back and forth between old and new code. This will go away in the final
> > submission.
> > 
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > Signed-off-by: Eugenio Pérez <[email-masked]>
> > Link: https://lore.kernel.org/r/[email-masked]
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > ---
> >   drivers/vhost/vhost.c | 297 +++++++++++++++++++++++++++++++++++++++++-
> >   drivers/vhost/vhost.h |  16 +++
> >   2 files changed, 312 insertions(+), 1 deletion(-)
> > 
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 96d9871fa0cb..105fc97af2c8 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -298,6 +298,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
> >   			   struct vhost_virtqueue *vq)
> >   {
> >   	vq->num = 1;
> > +	vq->ndescs = 0;
> >   	vq->desc = NULL;
> >   	vq->avail = NULL;
> >   	vq->used = NULL;
> > @@ -368,6 +369,9 @@ static int vhost_worker(void *data)
> >   static void vhost_vq_free_iovecs(struct vhost_virtqueue *vq)
> >   {
> > +	kfree(vq->descs);
> > +	vq->descs = NULL;
> > +	vq->max_descs = 0;
> >   	kfree(vq->indirect);
> >   	vq->indirect = NULL;
> >   	kfree(vq->log);
> > @@ -384,6 +388,10 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
> >   	for (i = 0; i < dev->nvqs; ++i) {
> >   		vq = dev->vqs[i];
> > +		vq->max_descs = dev->iov_limit;
> > +		vq->descs = kmalloc_array(vq->max_descs,
> > +					  sizeof(*vq->descs),
> > +					  GFP_KERNEL);
> >   		vq->indirect = kmalloc_array(UIO_MAXIOV,
> >   					     sizeof(*vq->indirect),
> >   					     GFP_KERNEL);
> > @@ -391,7 +399,7 @@ static long vhost_dev_alloc_iovecs(struct vhost_dev *dev)
> >   					GFP_KERNEL);
> >   		vq->heads = kmalloc_array(dev->iov_limit, sizeof(*vq->heads),
> >   					  GFP_KERNEL);
> > -		if (!vq->indirect || !vq->log || !vq->heads)
> > +		if (!vq->indirect || !vq->log || !vq->heads || !vq->descs)
> >   			goto err_nomem;
> >   	}
> >   	return 0;
> > @@ -2277,6 +2285,293 @@ int vhost_get_vq_desc(struct vhost_virtqueue *vq,
> >   }
> >   EXPORT_SYMBOL_GPL(vhost_get_vq_desc);
> > +static struct vhost_desc *peek_split_desc(struct vhost_virtqueue *vq)
> > +{
> > +	BUG_ON(!vq->ndescs);
> > +	return &vq->descs[vq->ndescs - 1];
> > +}
> > +
> > +static void pop_split_desc(struct vhost_virtqueue *vq)
> > +{
> > +	BUG_ON(!vq->ndescs);
> > +	--vq->ndescs;
> > +}
> > +
> > +#define VHOST_DESC_FLAGS (VRING_DESC_F_INDIRECT | VRING_DESC_F_WRITE | \
> > +			  VRING_DESC_F_NEXT)
> > +static int push_split_desc(struct vhost_virtqueue *vq, struct vring_desc *desc, u16 id)
> > +{
> > +	struct vhost_desc *h;
> > +
> > +	if (unlikely(vq->ndescs >= vq->max_descs))
> > +		return -EINVAL;
> > +	h = &vq->descs[vq->ndescs++];
> > +	h->addr = vhost64_to_cpu(vq, desc->addr);
> > +	h->len = vhost32_to_cpu(vq, desc->len);
> > +	h->flags = vhost16_to_cpu(vq, desc->flags) & VHOST_DESC_FLAGS;
> > +	h->id = id;
> > +
> > +	return 0;
> > +}
> > +
> > +static int fetch_indirect_descs(struct vhost_virtqueue *vq,
> > +				struct vhost_desc *indirect,
> > +				u16 head)
> > +{
> > +	struct vring_desc desc;
> > +	unsigned int i = 0, count, found = 0;
> > +	u32 len = indirect->len;
> > +	struct iov_iter from;
> > +	int ret;
> > +
> > +	/* Sanity check */
> > +	if (unlikely(len % sizeof desc)) {
> > +		vq_err(vq, "Invalid length in indirect descriptor: "
> > +		       "len 0x%llx not multiple of 0x%zx\n",
> > +		       (unsigned long long)len,
> > +		       sizeof desc);
> > +		return -EINVAL;
> > +	}
> > +
> > +	ret = translate_desc(vq, indirect->addr, len, vq->indirect,
> > +			     UIO_MAXIOV, VHOST_ACCESS_RO);
> > +	if (unlikely(ret < 0)) {
> > +		if (ret != -EAGAIN)
> > +			vq_err(vq, "Translation failure %d in indirect.\n", ret);
> > +		return ret;
> > +	}
> > +	iov_iter_init(&from, READ, vq->indirect, ret, len);
> > +
> > +	/* We will use the result as an address to read from, so most
> > +	 * architectures only need a compiler barrier here. */
> > +	read_barrier_depends();
> > +
> > +	count = len / sizeof desc;
> > +	/* Buffers are chained via a 16 bit next field, so
> > +	 * we can have at most 2^16 of these. */
> > +	if (unlikely(count > USHRT_MAX + 1)) {
> > +		vq_err(vq, "Indirect buffer length too big: %d\n",
> > +		       indirect->len);
> > +		return -E2BIG;
> > +	}
> > +	if (unlikely(vq->ndescs + count > vq->max_descs)) {
> > +		vq_err(vq, "Too many indirect + direct descs: %d + %d\n",
> > +		       vq->ndescs, indirect->len);
> > +		return -E2BIG;
> > +	}
> > +
> > +	do {
> > +		if (unlikely(++found > count)) {
> > +			vq_err(vq, "Loop detected: last one at %u "
> > +			       "indirect size %u\n",
> > +			       i, count);
> > +			return -EINVAL;
> > +		}
> > +		if (unlikely(!copy_from_iter_full(&desc, sizeof(desc), &from))) {
> > +			vq_err(vq, "Failed indirect descriptor: idx %d, %zx\n",
> > +			       i, (size_t)indirect->addr + i * sizeof desc);
> > +			return -EINVAL;
> > +		}
> > +		if (unlikely(desc.flags & cpu_to_vhost16(vq, VRING_DESC_F_INDIRECT))) {
> > +			vq_err(vq, "Nested indirect descriptor: idx %d, %zx\n",
> > +			       i, (size_t)indirect->addr + i * sizeof desc);
> > +			return -EINVAL;
> > +		}
> > +
> > +		push_split_desc(vq, &desc, head);
> 
> 
> The error is ignored.

See above:

     	if (unlikely(vq->ndescs + count > vq->max_descs)) 

So it can't fail here, we never fetch unless there's space.

I guess we can add a WARN_ON here.

> 
> > +	} while ((i = next_desc(vq, &desc)) != -1);
> > +	return 0;
> > +}
> > +
> > +static int fetch_descs(struct vhost_virtqueue *vq)
> > +{
> > +	unsigned int i, head, found = 0;
> > +	struct vhost_desc *last;
> > +	struct vring_desc desc;
> > +	__virtio16 avail_idx;
> > +	__virtio16 ring_head;
> > +	u16 last_avail_idx;
> > +	int ret;
> > +
> > +	/* Check it isn't doing very strange things with descriptor numbers. */
> > +	last_avail_idx = vq->last_avail_idx;
> > +
> > +	if (vq->avail_idx == vq->last_avail_idx) {
> > +		if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
> > +			vq_err(vq, "Failed to access avail idx at %p\n",
> > +				&vq->avail->idx);
> > +			return -EFAULT;
> > +		}
> > +		vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
> > +
> > +		if (unlikely((u16)(vq->avail_idx - last_avail_idx) > vq->num)) {
> > +			vq_err(vq, "Guest moved used index from %u to %u",
> > +				last_avail_idx, vq->avail_idx);
> > +			return -EFAULT;
> > +		}
> > +
> > +		/* If there's nothing new since last we looked, return
> > +		 * invalid.
> > +		 */
> > +		if (vq->avail_idx == last_avail_idx)
> > +			return vq->num;
> > +
> > +		/* Only get avail ring entries after they have been
> > +		 * exposed by guest.
> > +		 */
> > +		smp_rmb();
> > +	}
> > +
> > +	/* Grab the next descriptor number they're advertising */
> > +	if (unlikely(vhost_get_avail_head(vq, &ring_head, last_avail_idx))) {
> > +		vq_err(vq, "Failed to read head: idx %d address %p\n",
> > +		       last_avail_idx,
> > +		       &vq->avail->ring[last_avail_idx % vq->num]);
> > +		return -EFAULT;
> > +	}
> > +
> > +	head = vhost16_to_cpu(vq, ring_head);
> > +
> > +	/* If their number is silly, that's an error. */
> > +	if (unlikely(head >= vq->num)) {
> > +		vq_err(vq, "Guest says index %u > %u is available",
> > +		       head, vq->num);
> > +		return -EINVAL;
> > +	}
> > +
> > +	i = head;
> > +	do {
> > +		if (unlikely(i >= vq->num)) {
> > +			vq_err(vq, "Desc index is %u > %u, head = %u",
> > +			       i, vq->num, head);
> > +			return -EINVAL;
> > +		}
> > +		if (unlikely(++found > vq->num)) {
> > +			vq_err(vq, "Loop detected: last one at %u "
> > +			       "vq size %u head %u\n",
> > +			       i, vq->num, head);
> > +			return -EINVAL;
> > +		}
> > +		ret = vhost_get_desc(vq, &desc, i);
> > +		if (unlikely(ret)) {
> > +			vq_err(vq, "Failed to get descriptor: idx %d addr %p\n",
> > +			       i, vq->desc + i);
> > +			return -EFAULT;
> > +		}
> > +		ret = push_split_desc(vq, &desc, head);
> > +		if (unlikely(ret)) {
> > +			vq_err(vq, "Failed to save descriptor: idx %d\n", i);
> > +			return -EINVAL;
> > +		}
> > +	} while ((i = next_desc(vq, &desc)) != -1);
> > +
> > +	last = peek_split_desc(vq);
> > +	if (unlikely(last->flags & VRING_DESC_F_INDIRECT)) {
> > +		pop_split_desc(vq);
> > +		ret = fetch_indirect_descs(vq, last, head);
> 
> 
> Note that this means we don't supported chained indirect descriptors which
> complies the spec but we support this in vhost_get_vq_desc().

Well the spec says:
	A driver MUST NOT set both VIRTQ_DESC_F_INDIRECT and VIRTQ_DESC_F_NEXT in flags.

Did I miss anything?

> We probably need either fail early or just support that.
> 
> Thanks
> 
> 
> > +		if (unlikely(ret < 0)) {
> > +			if (ret != -EAGAIN)
> > +				vq_err(vq, "Failure detected "
> > +				       "in indirect descriptor at idx %d\n", head);
> > +			return ret;
> > +		}
> > +	}
> > +
> > +	/* Assume notifications from guest are disabled at this point,
> > +	 * if they aren't we would need to update avail_event index. */
> > +	BUG_ON(!(vq->used_flags & VRING_USED_F_NO_NOTIFY));
> > +
> > +	/* On success, increment avail index. */
> > +	vq->last_avail_idx++;
> > +
> > +	return 0;
> > +}
> > +
> > +/* This looks in the virtqueue and for the first available buffer, and converts
> > + * it to an iovec for convenient access.  Since descriptors consist of some
> > + * number of output then some number of input descriptors, it's actually two
> > + * iovecs, but we pack them into one and note how many of each there were.
> > + *
> > + * This function returns the descriptor number found, or vq->num (which is
> > + * never a valid descriptor number) if none was found.  A negative code is
> > + * returned on error. */
> > +int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
> > +		      struct iovec iov[], unsigned int iov_size,
> > +		      unsigned int *out_num, unsigned int *in_num,
> > +		      struct vhost_log *log, unsigned int *log_num)
> > +{
> > +	int ret = fetch_descs(vq);
> > +	int i;
> > +
> > +	if (ret)
> > +		return ret;
> > +
> > +	/* Now convert to IOV */
> > +	/* When we start there are none of either input nor output. */
> > +	*out_num = *in_num = 0;
> > +	if (unlikely(log))
> > +		*log_num = 0;
> > +
> > +	for (i = 0; i < vq->ndescs; ++i) {
> > +		unsigned iov_count = *in_num + *out_num;
> > +		struct vhost_desc *desc = &vq->descs[i];
> > +		int access;
> > +
> > +		if (desc->flags & ~VHOST_DESC_FLAGS) {
> > +			vq_err(vq, "Unexpected flags: 0x%x at descriptor id 0x%x\n",
> > +			       desc->flags, desc->id);
> > +			ret = -EINVAL;
> > +			goto err;
> > +		}
> > +		if (desc->flags & VRING_DESC_F_WRITE)
> > +			access = VHOST_ACCESS_WO;
> > +		else
> > +			access = VHOST_ACCESS_RO;
> > +		ret = translate_desc(vq, desc->addr,
> > +				     desc->len, iov + iov_count,
> > +				     iov_size - iov_count, access);
> > +		if (unlikely(ret < 0)) {
> > +			if (ret != -EAGAIN)
> > +				vq_err(vq, "Translation failure %d descriptor idx %d\n",
> > +					ret, i);
> > +			goto err;
> > +		}
> > +		if (access == VHOST_ACCESS_WO) {
> > +			/* If this is an input descriptor,
> > +			 * increment that count. */
> > +			*in_num += ret;
> > +			if (unlikely(log && ret)) {
> > +				log[*log_num].addr = desc->addr;
> > +				log[*log_num].len = desc->len;
> > +				++*log_num;
> > +			}
> > +		} else {
> > +			/* If it's an output descriptor, they're all supposed
> > +			 * to come before any input descriptors. */
> > +			if (unlikely(*in_num)) {
> > +				vq_err(vq, "Descriptor has out after in: "
> > +				       "idx %d\n", i);
> > +				ret = -EINVAL;
> > +				goto err;
> > +			}
> > +			*out_num += ret;
> > +		}
> > +
> > +		ret = desc->id;
> > +	}
> > +
> > +	vq->ndescs = 0;
> > +
> > +	return ret;
> > +
> > +err:
> > +	vhost_discard_vq_desc(vq, 1);
> > +	vq->ndescs = 0;
> > +
> > +	return ret;
> > +}
> > +EXPORT_SYMBOL_GPL(vhost_get_vq_desc_batch);
> > +
> >   /* Reverse the effect of vhost_get_vq_desc. Useful for error handling. */
> >   void vhost_discard_vq_desc(struct vhost_virtqueue *vq, int n)
> >   {
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index 60cab4c78229..0976a2853935 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -60,6 +60,13 @@ enum vhost_uaddr_type {
> >   	VHOST_NUM_ADDRS = 3,
> >   };
> > +struct vhost_desc {
> > +	u64 addr;
> > +	u32 len;
> > +	u16 flags; /* VRING_DESC_F_WRITE, VRING_DESC_F_NEXT */
> > +	u16 id;
> > +};
> > +
> >   /* The virtqueue structure describes a queue attached to a device. */
> >   struct vhost_virtqueue {
> >   	struct vhost_dev *dev;
> > @@ -71,6 +78,11 @@ struct vhost_virtqueue {
> >   	vring_avail_t __user *avail;
> >   	vring_used_t __user *used;
> >   	const struct vhost_iotlb_map *meta_iotlb[VHOST_NUM_ADDRS];
> > +
> > +	struct vhost_desc *descs;
> > +	int ndescs;
> > +	int max_descs;
> > +
> >   	struct file *kick;
> >   	struct eventfd_ctx *call_ctx;
> >   	struct eventfd_ctx *error_ctx;
> > @@ -175,6 +187,10 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
> >   bool vhost_vq_access_ok(struct vhost_virtqueue *vq);
> >   bool vhost_log_access_ok(struct vhost_dev *);
> > +int vhost_get_vq_desc_batch(struct vhost_virtqueue *,
> > +		      struct iovec iov[], unsigned int iov_count,
> > +		      unsigned int *out_num, unsigned int *in_num,
> > +		      struct vhost_log *log, unsigned int *log_num);
> >   int vhost_get_vq_desc(struct vhost_virtqueue *,
> >   		      struct iovec iov[], unsigned int iov_count,
> >   		      unsigned int *out_num, unsigned int *in_num,