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

[Yan Zhao]

On Fri, Jan 10, 2020 at 02:16:36AM +0800, Alex Williamson wrote:
> On Thu,  2 Jan 2020 20:02:17 -0500
> Yan Zhao <yan.y.zhao@xxxxxxxxx> 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@xxxxxxxxx>
> > Signed-off-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
> > ---
> >  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()?
>
ok. will rename the interface to vfio_dma_rw(). thanks :)

> > +{
> > +	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
>
Yes, thanks for pointing it out!

> > +
> > +	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,
>
you are right. Maybe I can first search dma with size of 1 and then
check the size of the found dma to perform a larger copy.

Thanks
Yan

> 
> > +		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,
>