Re: [PATCH 1/2] vfio: introduce vfio_iova_rw to read/write a range of IOVAs

[Alex Williamson <alex.williamson@redhat.com>]

On Thu,  2 Jan 2020 20:02:17 -0500
Yan Zhao <yan.y.zhao@intel.com> wrote:

> vfio_iova_rw will read/write a range of userspace memory (starting form
> device iova to iova + len -1) into a kenrel buffer without pinning the
> userspace memory.
> 
> TODO: vfio needs to mark the iova dirty if vfio_iova_rw(write) is
> called.
> 
> Cc: Kevin Tian <kevin.tian@intel.com>
> Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
> ---
>  drivers/vfio/vfio.c             | 45 ++++++++++++++++++
>  drivers/vfio/vfio_iommu_type1.c | 81 +++++++++++++++++++++++++++++++++
>  include/linux/vfio.h            |  5 ++
>  3 files changed, 131 insertions(+)
> 
> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> index c8482624ca34..36e91e647ed5 100644
> --- a/drivers/vfio/vfio.c
> +++ b/drivers/vfio/vfio.c
> @@ -1961,6 +1961,51 @@ int vfio_unpin_pages(struct device *dev, unsigned long *user_pfn, int npage)
>  }
>  EXPORT_SYMBOL(vfio_unpin_pages);
>  
> +/*
> + * Read/Write a range of userspace IOVAs for a device into/from a kernel
> + * buffer without pinning the userspace memory
> + * @dev [in]  : device
> + * @iova [in] : base IOVA of a userspace buffer
> + * @data [in] : pointer to kernel buffer
> + * @len [in]  : kernel buffer length
> + * @write     : indicate read or write
> + * Return error on failure or 0 on success.
> + */
> +int vfio_iova_rw(struct device *dev, unsigned long iova, void *data,
> +		   unsigned long len, bool write)

Shouldn't iova be a dma_addr_t and len be a size_t?  AIUI this function
performs the equivalent behavior of the device itself performing a DMA.
Hmm, should the interface be named vfio_dma_rw()?

> +{
> +	struct vfio_container *container;
> +	struct vfio_group *group;
> +	struct vfio_iommu_driver *driver;
> +	int ret = 0;
> +
> +	if (!dev || !data || len <= 0)
> +		return -EINVAL;
> +
> +	group = vfio_group_get_from_dev(dev);
> +	if (!group)
> +		return -ENODEV;
> +
> +	ret = vfio_group_add_container_user(group);
> +	if (ret)
> +		goto out;
> +
> +	container = group->container;
> +	driver = container->iommu_driver;
> +
> +	if (likely(driver && driver->ops->iova_rw))
> +		ret = driver->ops->iova_rw(container->iommu_data,
> +					   iova, data, len, write);
> +	else
> +		ret = -ENOTTY;
> +
> +	vfio_group_try_dissolve_container(group);
> +out:
> +	vfio_group_put(group);
> +	return ret;
> +}
> +EXPORT_SYMBOL(vfio_iova_rw);
> +
>  static int vfio_register_iommu_notifier(struct vfio_group *group,
>  					unsigned long *events,
>  					struct notifier_block *nb)
> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> index 2ada8e6cdb88..aee191077235 100644
> --- a/drivers/vfio/vfio_iommu_type1.c
> +++ b/drivers/vfio/vfio_iommu_type1.c
> @@ -27,6 +27,7 @@
>  #include <linux/iommu.h>
>  #include <linux/module.h>
>  #include <linux/mm.h>
> +#include <linux/mmu_context.h>
>  #include <linux/rbtree.h>
>  #include <linux/sched/signal.h>
>  #include <linux/sched/mm.h>
> @@ -2326,6 +2327,85 @@ static int vfio_iommu_type1_unregister_notifier(void *iommu_data,
>  	return blocking_notifier_chain_unregister(&iommu->notifier, nb);
>  }
>  
> +static int next_segment(unsigned long len, int offset)
> +{
> +	if (len > PAGE_SIZE - offset)
> +		return PAGE_SIZE - offset;
> +	else
> +		return len;
> +}
> +
> +static int vfio_iommu_type1_rw_iova_seg(struct vfio_iommu *iommu,
> +					  unsigned long iova, void *data,
> +					  unsigned long seg_len,
> +					  unsigned long offset,
> +					  bool write)
> +{
> +	struct mm_struct *mm;
> +	unsigned long vaddr;
> +	struct vfio_dma *dma;
> +	bool kthread = current->mm == NULL;
> +	int ret = 0;
> +
> +	dma = vfio_find_dma(iommu, iova, PAGE_SIZE);
> +	if (!dma)
> +		return -EINVAL;
> +
> +	mm = get_task_mm(dma->task);
> +
> +	if (!mm)
> +		return -ENODEV;
> +
> +	if (kthread)
> +		use_mm(mm);
> +	else if (current->mm != mm) {
> +		ret = -EINVAL;
> +		goto out;
> +	}
> +
> +	vaddr = dma->vaddr + iova - dma->iova + offset;

Parenthesis here would be useful and might prevent overflow, ie:

  dma->vaddr + (iova - dma->iova) + offset

> +
> +	ret = write ? __copy_to_user((void __user *)vaddr,
> +			data, seg_len) :
> +		__copy_from_user(data, (void __user *)vaddr,
> +				seg_len);
> +	if (ret)
> +		ret = -EFAULT;
> +
> +	if (kthread)
> +		unuse_mm(mm);
> +out:
> +	mmput(mm);
> +	return ret;
> +}
> +
> +static int vfio_iommu_type1_iova_rw(void *iommu_data, unsigned long iova,
> +				    void *data, unsigned long len, bool write)
> +{
> +	struct vfio_iommu *iommu = iommu_data;
> +	int offset = iova & ~PAGE_MASK;
> +	int seg_len;
> +	int ret = 0;
> +
> +	iova = iova & PAGE_MASK;
> +
> +	mutex_lock(&iommu->lock);
> +	while ((seg_len = next_segment(len, offset)) > 0) {
> +		ret = vfio_iommu_type1_rw_iova_seg(iommu, iova, data,
> +						   seg_len, offset, write);

Why do we need to split operations at page boundaries?  It seems really
inefficient that at each page crossing we need to lookup the vfio_dma
again (probably the same one), switch to the mm (probably the same one),
and perform another copy_{to,from}_user() when potentially have
everything we need to perform a larger copy.  Thanks,

Alex

> +		if (ret)
> +			break;
> +
> +		offset = 0;
> +		len -= seg_len;
> +		data += seg_len;
> +		iova += PAGE_SIZE;
> +	}
> +
> +	mutex_unlock(&iommu->lock);
> +	return ret;
> +}
> +
>  static const struct vfio_iommu_driver_ops vfio_iommu_driver_ops_type1 = {
>  	.name			= "vfio-iommu-type1",
>  	.owner			= THIS_MODULE,
> @@ -2338,6 +2418,7 @@ static const struct vfio_iommu_driver_ops vfio_iommu_driver_ops_type1 = {
>  	.unpin_pages		= vfio_iommu_type1_unpin_pages,
>  	.register_notifier	= vfio_iommu_type1_register_notifier,
>  	.unregister_notifier	= vfio_iommu_type1_unregister_notifier,
> +	.iova_rw		= vfio_iommu_type1_iova_rw,
>  };
>  
>  static int __init vfio_iommu_type1_init(void)
> diff --git a/include/linux/vfio.h b/include/linux/vfio.h
> index e42a711a2800..7bf18a31bbcf 100644
> --- a/include/linux/vfio.h
> +++ b/include/linux/vfio.h
> @@ -82,6 +82,8 @@ struct vfio_iommu_driver_ops {
>  					     struct notifier_block *nb);
>  	int		(*unregister_notifier)(void *iommu_data,
>  					       struct notifier_block *nb);
> +	int		(*iova_rw)(void *iommu_data, unsigned long iova,
> +				   void *data, unsigned long len, bool write);
>  };
>  
>  extern int vfio_register_iommu_driver(const struct vfio_iommu_driver_ops *ops);
> @@ -107,6 +109,9 @@ extern int vfio_pin_pages(struct device *dev, unsigned long *user_pfn,
>  extern int vfio_unpin_pages(struct device *dev, unsigned long *user_pfn,
>  			    int npage);
>  
> +extern int vfio_iova_rw(struct device *dev, unsigned long iova, void *data,
> +			unsigned long len, bool write);
> +
>  /* each type has independent events */
>  enum vfio_notify_type {
>  	VFIO_IOMMU_NOTIFY = 0,