Subject: Re: [PATCH RFC v1 2/2] vhost: batching fetches

From: "Michael S. Tsirkin" <[email-masked]>
Date: Sat, 12 Oct 2019 16:36:19 -0400

On Sat, Oct 12, 2019 at 03:30:52PM +0800, Jason Wang wrote:
> 
> On 2019/10/11 下午9:46, Michael S. Tsirkin wrote:
> > With this patch applied, new and old code perform identically.
> > 
> > Lots of extra optimizations are now possible, e.g.
> > we can fetch multiple heads with copy_from/to_user now.
> > We can get rid of maintaining the log array.  Etc etc.
> > 
> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > ---
> >   drivers/vhost/test.c  |  2 +-
> >   drivers/vhost/vhost.c | 50 ++++++++++++++++++++++++++++++++++++-------
> >   drivers/vhost/vhost.h |  4 +++-
> >   3 files changed, 46 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/vhost/test.c b/drivers/vhost/test.c
> > index 39a018a7af2d..e3a8e9db22cd 100644
> > --- a/drivers/vhost/test.c
> > +++ b/drivers/vhost/test.c
> > @@ -128,7 +128,7 @@ static int vhost_test_open(struct inode *inode, struct file *f)
> >   	dev = &n->dev;
> >   	vqs[VHOST_TEST_VQ] = &n->vqs[VHOST_TEST_VQ];
> >   	n->vqs[VHOST_TEST_VQ].handle_kick = handle_vq_kick;
> > -	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV,
> > +	vhost_dev_init(dev, vqs, VHOST_TEST_VQ_MAX, UIO_MAXIOV + 64,
> >   		       VHOST_TEST_PKT_WEIGHT, VHOST_TEST_WEIGHT);
> >   	f->private_data = n;
> > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > index 36661d6cb51f..aa383e847865 100644
> > --- a/drivers/vhost/vhost.c
> > +++ b/drivers/vhost/vhost.c
> > @@ -302,6 +302,7 @@ static void vhost_vq_reset(struct vhost_dev *dev,
> >   {
> >   	vq->num = 1;
> >   	vq->ndescs = 0;
> > +	vq->first_desc = 0;
> >   	vq->desc = NULL;
> >   	vq->avail = NULL;
> >   	vq->used = NULL;
> > @@ -390,6 +391,7 @@ 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->batch_descs = dev->iov_limit - UIO_MAXIOV;
> >   		vq->descs = kmalloc_array(vq->max_descs,
> >   					  sizeof(*vq->descs),
> >   					  GFP_KERNEL);
> > @@ -2366,6 +2368,8 @@ static void pop_split_desc(struct vhost_virtqueue *vq)
> >   	--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;
> > @@ -2375,7 +2379,7 @@ static int push_split_desc(struct vhost_virtqueue *vq, struct vring_desc *desc,
> >   	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);
> > +	h->flags = vhost16_to_cpu(vq, desc->flags) & VHOST_DESC_FLAGS;
> >   	h->id = id;
> >   	return 0;
> > @@ -2450,7 +2454,7 @@ static int fetch_indirect_descs(struct vhost_virtqueue *vq,
> >   	return 0;
> >   }
> > -static int fetch_descs(struct vhost_virtqueue *vq)
> > +static int fetch_buf(struct vhost_virtqueue *vq)
> >   {
> >   	struct vring_desc desc;
> >   	unsigned int i, head, found = 0;
> > @@ -2462,7 +2466,11 @@ static int fetch_descs(struct vhost_virtqueue *vq)
> >   	/* 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(vq->avail_idx == vq->last_avail_idx)) {
> > +		/* If we already have work to do, don't bother re-checking. */
> > +		if (likely(vq->ndescs))
> > +			return vq->num;
> > +
> >   		if (unlikely(vhost_get_avail_idx(vq, &avail_idx))) {
> >   			vq_err(vq, "Failed to access avail idx at %p\n",
> >   				&vq->avail->idx);
> > @@ -2541,6 +2549,24 @@ static int fetch_descs(struct vhost_virtqueue *vq)
> >   	return 0;
> >   }
> > +static int fetch_descs(struct vhost_virtqueue *vq)
> > +{
> > +	int ret = 0;
> > +
> > +	if (unlikely(vq->first_desc >= vq->ndescs)) {
> > +		vq->first_desc = 0;
> > +		vq->ndescs = 0;
> > +	}
> > +
> > +	if (vq->ndescs)
> > +		return 0;
> > +
> > +	while (!ret && vq->ndescs <= vq->batch_descs)
> > +		ret = fetch_buf(vq);
> 
> 
> It looks to me descriptor chaining might be broken here.

It should work because fetch_buf fetches a whole buf, following
the chain. Seems to work in a small test ... what issues do you see?

> 
> > +
> > +	return vq->ndescs ? 0 : ret;
> > +}
> > +
> >   /* 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
> > @@ -2562,6 +2588,8 @@ int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
> >   	if (ret)
> >   		return ret;
> > +	/* Note: indirect descriptors are not batched */
> > +	/* TODO: batch up to a limit */
> >   	last = peek_split_desc(vq);
> >   	id = last->id;
> > @@ -2584,12 +2612,12 @@ int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
> >   	if (unlikely(log))
> >   		*log_num = 0;
> > -	for (i = 0; i < vq->ndescs; ++i) {
> > +	for (i = vq->first_desc; i < vq->ndescs; ++i) {
> >   		unsigned iov_count = *in_num + *out_num;
> >   		struct vhost_desc *desc = &vq->descs[i];
> >   		int access;
> > -		if (desc->flags & ~(VRING_DESC_F_INDIRECT | VRING_DESC_F_WRITE)) {
> > +		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;
> > @@ -2628,15 +2656,21 @@ int vhost_get_vq_desc_batch(struct vhost_virtqueue *vq,
> >   			}
> >   			*out_num += ret;
> >   		}
> > +
> > +		ret = desc->id;
> > +
> > +		if (!(desc->flags & VRING_DESC_F_NEXT))
> > +			break;
> >   	}
> 
> 
> What happens if we reach vq->ndescs but VRING_DESC_F_NEXT is still set?
> 
> Thanks

This can't happen: descriptors are pushed by push_split_desc each time
we go through a loop in fetch_buf. The only way to exit the loop
with return code 0 is if next_desc return -1 that is when VRING_DESC_F_NEXT
is clear.

But it's a good idea to add a BUG_ON here, I'll do it in the next version.

> 
> > -	ret = id;
> > -	vq->ndescs = 0;
> > +	vq->first_desc = i + 1;
> >   	return ret;
> >   err:
> > -	vhost_discard_vq_desc(vq, 1);
> > +	for (i = vq->first_desc; i < vq->ndescs; ++i)
> > +		if (!(desc->flags & VRING_DESC_F_NEXT))
> > +			vhost_discard_vq_desc(vq, 1);
> >   	vq->ndescs = 0;
> >   	return ret;
> > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > index 1724f61b6c2d..8b88e0c903da 100644
> > --- a/drivers/vhost/vhost.h
> > +++ b/drivers/vhost/vhost.h
> > @@ -100,7 +100,9 @@ struct vhost_virtqueue {
> >   	struct vhost_desc *descs;
> >   	int ndescs;
> > +	int first_desc;
> >   	int max_descs;
> > +	int batch_descs;
> >   	const struct vhost_umem_node *meta_iotlb[VHOST_NUM_ADDRS];
> >   	struct file *kick;
> > @@ -245,7 +247,7 @@ ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
> >   int vhost_init_device_iotlb(struct vhost_dev *d, bool enabled);
> >   #define vq_err(vq, fmt, ...) do {                                  \
> > -		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \
> > +		pr_err(pr_fmt(fmt), ##__VA_ARGS__);       \
> >   		if ((vq)->error_ctx)                               \
> >   				eventfd_signal((vq)->error_ctx, 1);\
> >   	} while (0)