Linux Virtualization - Sep 2017 - [PATCH net-next RFC 2/5] vhost: introduce helper to prefetch desc index

On Fri, Sep 22, 2017 at 04:02:32PM +0800, Jason Wang
wrote:
> This patch introduces vhost_prefetch_desc_indices() which could batch
> descriptor indices fetching and used ring updating. This intends to
> reduce the cache misses of indices fetching and updating and reduce
> cache line bounce when virtqueue is almost full. copy_to_user() was
> used in order to benefit from modern cpus that support fast string
> copy. Batched virtqueue processing will be the first user.
> 
> Signed-off-by: Jason Wang <jasowang at redhat.com>
> ---
>  drivers/vhost/vhost.c | 55
+++++++++++++++++++++++++++++++++++++++++++++++++++
>  drivers/vhost/vhost.h |  3 +++
>  2 files changed, 58 insertions(+)
> 
> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> index f87ec75..8424166d 100644
> --- a/drivers/vhost/vhost.c
> +++ b/drivers/vhost/vhost.c
> @@ -2437,6 +2437,61 @@ struct vhost_msg_node *vhost_dequeue_msg(struct
vhost_dev *dev,
>  }
>  EXPORT_SYMBOL_GPL(vhost_dequeue_msg);
>  
> +int vhost_prefetch_desc_indices(struct vhost_virtqueue *vq,
> +				struct vring_used_elem *heads,
> +				u16 num, bool used_update)
why do you need to combine used update with prefetch?
> +{
> +	int ret, ret2;
> +	u16 last_avail_idx, last_used_idx, total, copied;
> +	__virtio16 avail_idx;
> +	struct vring_used_elem __user *used;
> +	int i;
> +
> +	if (unlikely(vhost_get_avail(vq, avail_idx, &vq->avail->idx)))
{
> +		vq_err(vq, "Failed to access avail idx at %p\n",
> +		       &vq->avail->idx);
> +		return -EFAULT;
> +	}
> +	last_avail_idx = vq->last_avail_idx & (vq->num - 1);
> +	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
> +	total = vq->avail_idx - vq->last_avail_idx;
> +	ret = total = min(total, num);
> +
> +	for (i = 0; i < ret; i++) {
> +		ret2 = vhost_get_avail(vq, heads[i].id,
> +				      &vq->avail->ring[last_avail_idx]);
> +		if (unlikely(ret2)) {
> +			vq_err(vq, "Failed to get descriptors\n");
> +			return -EFAULT;
> +		}
> +		last_avail_idx = (last_avail_idx + 1) & (vq->num - 1);
> +	}
> +
> +	if (!used_update)
> +		return ret;
> +
> +	last_used_idx = vq->last_used_idx & (vq->num - 1);
> +	while (total) {
> +		copied = min((u16)(vq->num - last_used_idx), total);
> +		ret2 = vhost_copy_to_user(vq,
> +					  &vq->used->ring[last_used_idx],
> +					  &heads[ret - total],
> +					  copied * sizeof(*used));
> +
> +		if (unlikely(ret2)) {
> +			vq_err(vq, "Failed to update used ring!\n");
> +			return -EFAULT;
> +		}
> +
> +		last_used_idx = 0;
> +		total -= copied;
> +	}
> +
> +	/* Only get avail ring entries after they have been exposed by guest. */
> +	smp_rmb();
Barrier before return is a very confusing API. I guess it's designed to
be used in a specific way to make it necessary - but what is it?

> +	return ret;
> +}
> +EXPORT_SYMBOL(vhost_prefetch_desc_indices);
>  
>  static int __init vhost_init(void)
>  {
> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> index 39ff897..16c2cb6 100644
> --- a/drivers/vhost/vhost.h
> +++ b/drivers/vhost/vhost.h
> @@ -228,6 +228,9 @@ ssize_t vhost_chr_read_iter(struct vhost_dev *dev,
struct iov_iter *to,
>  ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
>  			     struct iov_iter *from);
>  int vhost_init_device_iotlb(struct vhost_dev *d, bool enabled);
> +int vhost_prefetch_desc_indices(struct vhost_virtqueue *vq,
> +				struct vring_used_elem *heads,
> +				u16 num, bool used_update);
>  
>  #define vq_err(vq, fmt, ...) do {                                  \
>  		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \
> -- 
> 2.7.4

On 2017?09?27? 03:19, Michael S. Tsirkin wrote:
> On Fri, Sep 22, 2017 at 04:02:32PM +0800, Jason Wang wrote:
>> This patch introduces vhost_prefetch_desc_indices() which could batch
>> descriptor indices fetching and used ring updating. This intends to
>> reduce the cache misses of indices fetching and updating and reduce
>> cache line bounce when virtqueue is almost full. copy_to_user() was
>> used in order to benefit from modern cpus that support fast string
>> copy. Batched virtqueue processing will be the first user.
>>
>> Signed-off-by: Jason Wang <jasowang at redhat.com>
>> ---
>>   drivers/vhost/vhost.c | 55
+++++++++++++++++++++++++++++++++++++++++++++++++++
>>   drivers/vhost/vhost.h |  3 +++
>>   2 files changed, 58 insertions(+)
>>
>> diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
>> index f87ec75..8424166d 100644
>> --- a/drivers/vhost/vhost.c
>> +++ b/drivers/vhost/vhost.c
>> @@ -2437,6 +2437,61 @@ struct vhost_msg_node *vhost_dequeue_msg(struct
vhost_dev *dev,
>>   }
>>   EXPORT_SYMBOL_GPL(vhost_dequeue_msg);
>>   
>> +int vhost_prefetch_desc_indices(struct vhost_virtqueue *vq,
>> +				struct vring_used_elem *heads,
>> +				u16 num, bool used_update)
> why do you need to combine used update with prefetch?
For better performance and I believe we don't care about the overhead 
when we meet errors in tx.
>
>> +{
>> +	int ret, ret2;
>> +	u16 last_avail_idx, last_used_idx, total, copied;
>> +	__virtio16 avail_idx;
>> +	struct vring_used_elem __user *used;
>> +	int i;
>> +
>> +	if (unlikely(vhost_get_avail(vq, avail_idx,
&vq->avail->idx))) {
>> +		vq_err(vq, "Failed to access avail idx at %p\n",
>> +		       &vq->avail->idx);
>> +		return -EFAULT;
>> +	}
>> +	last_avail_idx = vq->last_avail_idx & (vq->num - 1);
>> +	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
>> +	total = vq->avail_idx - vq->last_avail_idx;
>> +	ret = total = min(total, num);
>> +
>> +	for (i = 0; i < ret; i++) {
>> +		ret2 = vhost_get_avail(vq, heads[i].id,
>> +				      &vq->avail->ring[last_avail_idx]);
>> +		if (unlikely(ret2)) {
>> +			vq_err(vq, "Failed to get descriptors\n");
>> +			return -EFAULT;
>> +		}
>> +		last_avail_idx = (last_avail_idx + 1) & (vq->num - 1);
>> +	}
>> +
>> +	if (!used_update)
>> +		return ret;
>> +
>> +	last_used_idx = vq->last_used_idx & (vq->num - 1);
>> +	while (total) {
>> +		copied = min((u16)(vq->num - last_used_idx), total);
>> +		ret2 = vhost_copy_to_user(vq,
>> +					  &vq->used->ring[last_used_idx],
>> +					  &heads[ret - total],
>> +					  copied * sizeof(*used));
>> +
>> +		if (unlikely(ret2)) {
>> +			vq_err(vq, "Failed to update used ring!\n");
>> +			return -EFAULT;
>> +		}
>> +
>> +		last_used_idx = 0;
>> +		total -= copied;
>> +	}
>> +
>> +	/* Only get avail ring entries after they have been exposed by guest.
*/
>> +	smp_rmb();
> Barrier before return is a very confusing API. I guess it's designed to
> be used in a specific way to make it necessary - but what is it?
Looks like a and we need do this after reading avail_idx.

Thanks
>
>
>> +	return ret;
>> +}
>> +EXPORT_SYMBOL(vhost_prefetch_desc_indices);
>>   
>>   static int __init vhost_init(void)
>>   {
>> diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
>> index 39ff897..16c2cb6 100644
>> --- a/drivers/vhost/vhost.h
>> +++ b/drivers/vhost/vhost.h
>> @@ -228,6 +228,9 @@ ssize_t vhost_chr_read_iter(struct vhost_dev *dev,
struct iov_iter *to,
>>   ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
>>   			     struct iov_iter *from);
>>   int vhost_init_device_iotlb(struct vhost_dev *d, bool enabled);
>> +int vhost_prefetch_desc_indices(struct vhost_virtqueue *vq,
>> +				struct vring_used_elem *heads,
>> +				u16 num, bool used_update);
>>   
>>   #define vq_err(vq, fmt, ...) do {                                  \
>>   		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \
>> -- 
>> 2.7.4

On Wed, Sep 27, 2017 at 08:35:47AM +0800, Jason Wang
wrote:
> 
> 
> On 2017?09?27? 03:19, Michael S. Tsirkin wrote:
> > On Fri, Sep 22, 2017 at 04:02:32PM +0800, Jason Wang wrote:
> > > This patch introduces vhost_prefetch_desc_indices() which could
batch
> > > descriptor indices fetching and used ring updating. This intends
to
> > > reduce the cache misses of indices fetching and updating and
reduce
> > > cache line bounce when virtqueue is almost full. copy_to_user()
was
> > > used in order to benefit from modern cpus that support fast
string
> > > copy. Batched virtqueue processing will be the first user.
> > > 
> > > Signed-off-by: Jason Wang <jasowang at redhat.com>
> > > ---
> > >   drivers/vhost/vhost.c | 55
+++++++++++++++++++++++++++++++++++++++++++++++++++
> > >   drivers/vhost/vhost.h |  3 +++
> > >   2 files changed, 58 insertions(+)
> > > 
> > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c
> > > index f87ec75..8424166d 100644
> > > --- a/drivers/vhost/vhost.c
> > > +++ b/drivers/vhost/vhost.c
> > > @@ -2437,6 +2437,61 @@ struct vhost_msg_node
*vhost_dequeue_msg(struct vhost_dev *dev,
> > >   }
> > >   EXPORT_SYMBOL_GPL(vhost_dequeue_msg);
> > > +int vhost_prefetch_desc_indices(struct vhost_virtqueue *vq,
> > > +				struct vring_used_elem *heads,
> > > +				u16 num, bool used_update)
> > why do you need to combine used update with prefetch?
> 
> For better performance

Why is sticking a branch in there better than requesting the update
conditionally from the caller?


> and I believe we don't care about the overhead when
> we meet errors in tx.
That's a separate question, I do not really understand how
you can fetch a descriptor and update the used ring at the same
time. This allows the guest to overwrite the buffer.
I might be misunderstanding what is going on here though.

> > 
> > > +{
> > > +	int ret, ret2;
> > > +	u16 last_avail_idx, last_used_idx, total, copied;
> > > +	__virtio16 avail_idx;
> > > +	struct vring_used_elem __user *used;
> > > +	int i;
> > > +
> > > +	if (unlikely(vhost_get_avail(vq, avail_idx,
&vq->avail->idx))) {
> > > +		vq_err(vq, "Failed to access avail idx at %p\n",
> > > +		       &vq->avail->idx);
> > > +		return -EFAULT;
> > > +	}
> > > +	last_avail_idx = vq->last_avail_idx & (vq->num - 1);
> > > +	vq->avail_idx = vhost16_to_cpu(vq, avail_idx);
> > > +	total = vq->avail_idx - vq->last_avail_idx;
> > > +	ret = total = min(total, num);
> > > +
> > > +	for (i = 0; i < ret; i++) {
> > > +		ret2 = vhost_get_avail(vq, heads[i].id,
> > > +				      &vq->avail->ring[last_avail_idx]);
> > > +		if (unlikely(ret2)) {
> > > +			vq_err(vq, "Failed to get descriptors\n");
> > > +			return -EFAULT;
> > > +		}
> > > +		last_avail_idx = (last_avail_idx + 1) & (vq->num - 1);
> > > +	}
> > > +
> > > +	if (!used_update)
> > > +		return ret;
> > > +
> > > +	last_used_idx = vq->last_used_idx & (vq->num - 1);
> > > +	while (total) {
> > > +		copied = min((u16)(vq->num - last_used_idx), total);
> > > +		ret2 = vhost_copy_to_user(vq,
> > > +					  &vq->used->ring[last_used_idx],
> > > +					  &heads[ret - total],
> > > +					  copied * sizeof(*used));
> > > +
> > > +		if (unlikely(ret2)) {
> > > +			vq_err(vq, "Failed to update used ring!\n");
> > > +			return -EFAULT;
> > > +		}
> > > +
> > > +		last_used_idx = 0;
> > > +		total -= copied;
> > > +	}
> > > +
> > > +	/* Only get avail ring entries after they have been exposed by
guest. */
> > > +	smp_rmb();
> > Barrier before return is a very confusing API. I guess it's
designed to
> > be used in a specific way to make it necessary - but what is it?
> 
> Looks like a and we need do this after reading avail_idx.
> 
> Thanks
> 
> > 
> > 
> > > +	return ret;
> > > +}
> > > +EXPORT_SYMBOL(vhost_prefetch_desc_indices);
> > >   static int __init vhost_init(void)
> > >   {
> > > diff --git a/drivers/vhost/vhost.h b/drivers/vhost/vhost.h
> > > index 39ff897..16c2cb6 100644
> > > --- a/drivers/vhost/vhost.h
> > > +++ b/drivers/vhost/vhost.h
> > > @@ -228,6 +228,9 @@ ssize_t vhost_chr_read_iter(struct vhost_dev
*dev, struct iov_iter *to,
> > >   ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
> > >   			     struct iov_iter *from);
> > >   int vhost_init_device_iotlb(struct vhost_dev *d, bool enabled);
> > > +int vhost_prefetch_desc_indices(struct vhost_virtqueue *vq,
> > > +				struct vring_used_elem *heads,
> > > +				u16 num, bool used_update);
> > >   #define vq_err(vq, fmt, ...) do {                              
\
> > >   		pr_debug(pr_fmt(fmt), ##__VA_ARGS__);       \
> > > -- 
> > > 2.7.4