Re: [PATCH RFC 4/5] vhost-vdpa: support IOTLB batching hints

["Michael S. Tsirkin" <mst@redhat.com>]

On Thu, Jun 18, 2020 at 01:56:25PM +0800, Jason Wang wrote:
> This patches extend the vhost IOTLB API to accept batch updating hints
> form userspace. When userspace wants update the device IOTLB in a
> batch, it may do:
> 
> 1) Write vhost_iotlb_msg with VHOST_IOTLB_BATCH_BEGIN flag
> 2) Perform a batch of IOTLB updating via VHOST_IOTLB_UPDATE/INVALIDATE
> 3) Write vhost_iotlb_msg with VHOST_IOTLB_BATCH_END flag

As long as we are extending the interface,
is there some way we could cut down the number of system calls needed
here?


> 
> Vhost-vdpa may decide to batch the IOMMU/IOTLB updating in step 3 when
> vDPA device support set_map() ops. This is useful for the vDPA device
> that want to know all the mappings to tweak their own DMA translation
> logic.
> 
> For vDPA device that doesn't require set_map(), no behavior changes.
> 
> This capability is advertised via VHOST_BACKEND_F_IOTLB_BATCH capability.
> 
> Signed-off-by: Jason Wang <jasowang@redhat.com>
> ---
>  drivers/vhost/vdpa.c             | 30 +++++++++++++++++++++++-------
>  include/uapi/linux/vhost.h       |  2 ++
>  include/uapi/linux/vhost_types.h |  7 +++++++
>  3 files changed, 32 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/vhost/vdpa.c b/drivers/vhost/vdpa.c
> index 453057421f80..8f624bbafee7 100644
> --- a/drivers/vhost/vdpa.c
> +++ b/drivers/vhost/vdpa.c
> @@ -56,7 +56,9 @@ enum {
>  };
>  
>  enum {
> -	VHOST_VDPA_BACKEND_FEATURES = (1ULL << VHOST_BACKEND_F_IOTLB_MSG_V2)
> +	VHOST_VDPA_BACKEND_FEATURES =
> +	(1ULL << VHOST_BACKEND_F_IOTLB_MSG_V2) |
> +	(1ULL << VHOST_BACKEND_F_IOTLB_BATCH),
>  };
>  
>  /* Currently, only network backend w/o multiqueue is supported. */
> @@ -77,6 +79,7 @@ struct vhost_vdpa {
>  	int virtio_id;
>  	int minor;
>  	struct eventfd_ctx *config_ctx;
> +	int in_batch;
>  };
>  
>  static DEFINE_IDA(vhost_vdpa_ida);
> @@ -125,6 +128,7 @@ static void vhost_vdpa_reset(struct vhost_vdpa *v)
>  	const struct vdpa_config_ops *ops = vdpa->config;
>  
>  	ops->set_status(vdpa, 0);
> +	v->in_batch = 0;
>  }
>  
>  static long vhost_vdpa_get_device_id(struct vhost_vdpa *v, u8 __user *argp)
> @@ -540,9 +544,10 @@ static int vhost_vdpa_map(struct vhost_vdpa *v,
>  
>  	if (ops->dma_map)
>  		r = ops->dma_map(vdpa, iova, size, pa, perm);
> -	else if (ops->set_map)
> -		r = ops->set_map(vdpa, dev->iotlb);
> -	else
> +	else if (ops->set_map) {
> +		if (!v->in_batch)
> +			r = ops->set_map(vdpa, dev->iotlb);
> +	} else
>  		r = iommu_map(v->domain, iova, pa, size,
>  			      perm_to_iommu_flags(perm));
>  
> @@ -559,9 +564,10 @@ static void vhost_vdpa_unmap(struct vhost_vdpa *v, u64 iova, u64 size)
>  
>  	if (ops->dma_map)
>  		ops->dma_unmap(vdpa, iova, size);
> -	else if (ops->set_map)
> -		ops->set_map(vdpa, dev->iotlb);
> -	else
> +	else if (ops->set_map) {
> +		if (!v->in_batch)
> +			ops->set_map(vdpa, dev->iotlb);
> +	} else
>  		iommu_unmap(v->domain, iova, size);
>  }
>  
> @@ -655,6 +661,8 @@ static int vhost_vdpa_process_iotlb_msg(struct vhost_dev *dev,
>  					struct vhost_iotlb_msg *msg)
>  {
>  	struct vhost_vdpa *v = container_of(dev, struct vhost_vdpa, vdev);
> +	struct vdpa_device *vdpa = v->vdpa;
> +	const struct vdpa_config_ops *ops = vdpa->config;
>  	int r = 0;
>  
>  	r = vhost_dev_check_owner(dev);
> @@ -668,6 +676,14 @@ static int vhost_vdpa_process_iotlb_msg(struct vhost_dev *dev,
>  	case VHOST_IOTLB_INVALIDATE:
>  		vhost_vdpa_unmap(v, msg->iova, msg->size);
>  		break;
> +	case VHOST_IOTLB_BATCH_BEGIN:
> +		v->in_batch = true;
> +		break;
> +	case VHOST_IOTLB_BATCH_END:
> +		if (v->in_batch && ops->set_map)
> +			ops->set_map(vdpa, dev->iotlb);
> +		v->in_batch = false;
> +		break;
>  	default:
>  		r = -EINVAL;
>  		break;
> diff --git a/include/uapi/linux/vhost.h b/include/uapi/linux/vhost.h
> index 0c2349612e77..565da96f55d5 100644
> --- a/include/uapi/linux/vhost.h
> +++ b/include/uapi/linux/vhost.h
> @@ -91,6 +91,8 @@
>  
>  /* Use message type V2 */
>  #define VHOST_BACKEND_F_IOTLB_MSG_V2 0x1
> +/* IOTLB can accpet batching hints */

typo

> +#define VHOST_BACKEND_F_IOTLB_BATCH  0x2
>  
>  #define VHOST_SET_BACKEND_FEATURES _IOW(VHOST_VIRTIO, 0x25, __u64)
>  #define VHOST_GET_BACKEND_FEATURES _IOR(VHOST_VIRTIO, 0x26, __u64)
> diff --git a/include/uapi/linux/vhost_types.h b/include/uapi/linux/vhost_types.h
> index 669457ce5c48..5c12faffdde9 100644
> --- a/include/uapi/linux/vhost_types.h
> +++ b/include/uapi/linux/vhost_types.h
> @@ -60,6 +60,13 @@ struct vhost_iotlb_msg {
>  #define VHOST_IOTLB_UPDATE         2
>  #define VHOST_IOTLB_INVALIDATE     3
>  #define VHOST_IOTLB_ACCESS_FAIL    4
> +/* VHOST_IOTLB_BATCH_BEGIN is a hint that userspace will update
> + * several mappings afterwards. VHOST_IOTLB_BATCH_END is a hint that
> + * userspace had finished the mapping updating.


Well not just hints - in fact updates do not take place
until _END.

How about:

/* VHOST_IOTLB_BATCH_BEGIN and VHOST_IOTLB_BATCH_END allow modifying
 * multiple mappings in one go: beginning with
 * VHOST_IOTLB_BATCH_BEGIN, followed by any number of
   VHOST_IOTLB_UPDATE messages, and ending with VHOST_IOTLB_BATCH_END.
 */


> When those two flags
> + * were set, kernel will ignore the rest fileds of the IOTLB message.

how about:

when one of these two values is used as the message type, the
rest of the fields in the message are ignored.

> + */
> +#define VHOST_IOTLB_BATCH_BEGIN    5
> +#define VHOST_IOTLB_BATCH_END      6
>  	__u8 type;
>  };
>  
> -- 
> 2.20.1