[RFC,v1,2/2] vhost: batching fetches
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Message ID 20191011134358.16912-3-mst@redhat.com
State New
Headers show
Series
  • vhost: ring format independence
Related show

Commit Message

Michael S. Tsirkin Oct. 11, 2019, 1:46 p.m. UTC
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 <mst@redhat.com>
---
 drivers/vhost/test.c  |  2 +-
 drivers/vhost/vhost.c | 50 ++++++++++++++++++++++++++++++++++++-------
 drivers/vhost/vhost.h |  4 +++-
 3 files changed, 46 insertions(+), 10 deletions(-)

Comments

Jason Wang Oct. 12, 2019, 7:30 a.m. UTC | #1
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 <mst@redhat.com>
> ---
>   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.


> +
> +	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


>   
> -	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)
Michael S. Tsirkin Oct. 12, 2019, 8:36 p.m. UTC | #2
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 <mst@redhat.com>
> > ---
> >   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)
Jason Wang Oct. 14, 2019, 1:47 a.m. UTC | #3
On 2019/10/13 上午4:36, Michael S. Tsirkin wrote:
> 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 <mst@redhat.com>
>>> ---
>>>    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?


Consider the case when fetch_buf return -EINVAL when push_split_desc() 
fail. This means we only have partial descriptors.

Do we need decreasing last_avail_idx and vq->ndescs here? Or having a 
head,tail instead of first_desc?

Thanks


>
>>> +
>>> +	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)

Patch
diff mbox series

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);
+
+	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;
 	}
 
-	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)