Re: [PATCH v10 10/19] vfio iommu: Add blocking notifier to notify DMA_UNMAP

[Alex Williamson]

On Sat, 29 Oct 2016 01:32:35 +0530
Kirti Wankhede <kwankhede@xxxxxxxxxx> wrote:

> On 10/28/2016 6:10 PM, Alex Williamson wrote:
> > On Fri, 28 Oct 2016 15:33:58 +0800
> > Jike Song <jike.song@xxxxxxxxx> wrote:
> >   
> >> On 10/27/2016 05:29 AM, Kirti Wankhede wrote:  
> >>> Added blocking notifier to IOMMU TYPE1 driver to notify vendor drivers
> >>> about DMA_UNMAP.
> >>> Exported two APIs vfio_register_notifier() and vfio_unregister_notifier().
> >>> Vendor driver should register notifer using these APIs.
> >>> Vendor driver should use VFIO_IOMMU_NOTIFY_DMA_UNMAP action to invalidate
> >>> mappings.
> >>>
> >>> Signed-off-by: Kirti Wankhede <kwankhede@xxxxxxxxxx>
> >>> Signed-off-by: Neo Jia <cjia@xxxxxxxxxx>
> >>> Change-Id: I5910d0024d6be87f3e8d3e0ca0eaeaaa0b17f271
> >>> ---
> >>>  drivers/vfio/vfio.c             | 73 +++++++++++++++++++++++++++++++++
> >>>  drivers/vfio/vfio_iommu_type1.c | 89 ++++++++++++++++++++++++++++++++++++-----
> >>>  include/linux/vfio.h            | 11 +++++
> >>>  3 files changed, 163 insertions(+), 10 deletions(-)
> >>>
> >>> diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
> >>> index 28b50ca14c52..ff05ac6b1e90 100644
> >>> --- a/drivers/vfio/vfio.c
> >>> +++ b/drivers/vfio/vfio.c
> >>> @@ -1891,6 +1891,79 @@ err_unpin_pages:
> >>>  }
> >>>  EXPORT_SYMBOL(vfio_unpin_pages);
> >>>  
> >>> +int vfio_register_notifier(struct device *dev, struct notifier_block *nb)
> >>> +{
> >>> +	struct vfio_container *container;
> >>> +	struct vfio_group *group;
> >>> +	struct vfio_iommu_driver *driver;
> >>> +	ssize_t ret;
> >>> +
> >>> +	if (!dev || !nb)
> >>> +		return -EINVAL;
> >>> +
> >>> +	group = vfio_group_get_from_dev(dev);
> >>> +	if (IS_ERR(group))
> >>> +		return PTR_ERR(group);
> >>> +
> >>> +	ret = vfio_group_add_container_user(group);
> >>> +	if (ret)
> >>> +		goto err_register_nb;
> >>> +
> >>> +	container = group->container;
> >>> +	down_read(&container->group_lock);
> >>> +
> >>> +	driver = container->iommu_driver;
> >>> +	if (likely(driver && driver->ops->register_notifier))
> >>> +		ret = driver->ops->register_notifier(container->iommu_data, nb);
> >>> +	else
> >>> +		ret = -EINVAL;
> >>> +
> >>> +	up_read(&container->group_lock);
> >>> +	vfio_group_try_dissolve_container(group);
> >>> +
> >>> +err_register_nb:
> >>> +	vfio_group_put(group);
> >>> +	return ret;
> >>> +}
> >>> +EXPORT_SYMBOL(vfio_register_notifier);
> >>> +
> >>> +int vfio_unregister_notifier(struct device *dev, struct notifier_block *nb)
> >>> +{
> >>> +	struct vfio_container *container;
> >>> +	struct vfio_group *group;
> >>> +	struct vfio_iommu_driver *driver;
> >>> +	ssize_t ret;
> >>> +
> >>> +	if (!dev || !nb)
> >>> +		return -EINVAL;
> >>> +
> >>> +	group = vfio_group_get_from_dev(dev);
> >>> +	if (IS_ERR(group))
> >>> +		return PTR_ERR(group);
> >>> +
> >>> +	ret = vfio_group_add_container_user(group);
> >>> +	if (ret)
> >>> +		goto err_unregister_nb;
> >>> +
> >>> +	container = group->container;
> >>> +	down_read(&container->group_lock);
> >>> +
> >>> +	driver = container->iommu_driver;
> >>> +	if (likely(driver && driver->ops->unregister_notifier))
> >>> +		ret = driver->ops->unregister_notifier(container->iommu_data,
> >>> +						       nb);
> >>> +	else
> >>> +		ret = -EINVAL;
> >>> +
> >>> +	up_read(&container->group_lock);
> >>> +	vfio_group_try_dissolve_container(group);
> >>> +
> >>> +err_unregister_nb:
> >>> +	vfio_group_put(group);
> >>> +	return ret;
> >>> +}
> >>> +EXPORT_SYMBOL(vfio_unregister_notifier);
> >>> +
> >>>  /**
> >>>   * Module/class support
> >>>   */
> >>> diff --git a/drivers/vfio/vfio_iommu_type1.c b/drivers/vfio/vfio_iommu_type1.c
> >>> index 5add11a147e1..a4bd331ac0fd 100644
> >>> --- a/drivers/vfio/vfio_iommu_type1.c
> >>> +++ b/drivers/vfio/vfio_iommu_type1.c
> >>> @@ -37,6 +37,7 @@
> >>>  #include <linux/vfio.h>
> >>>  #include <linux/workqueue.h>
> >>>  #include <linux/mdev.h>
> >>> +#include <linux/notifier.h>
> >>>  
> >>>  #define DRIVER_VERSION  "0.2"
> >>>  #define DRIVER_AUTHOR   "Alex Williamson <alex.williamson@xxxxxxxxxx>"
> >>> @@ -59,6 +60,7 @@ struct vfio_iommu {
> >>>  	struct vfio_domain	*external_domain; /* domain for external user */
> >>>  	struct mutex		lock;
> >>>  	struct rb_root		dma_list;
> >>> +	struct blocking_notifier_head notifier;
> >>>  	bool			v2;
> >>>  	bool			nesting;
> >>>  };
> >>> @@ -549,7 +551,8 @@ static long vfio_iommu_type1_pin_pages(void *iommu_data,
> >>>  
> >>>  	mutex_lock(&iommu->lock);
> >>>  
> >>> -	if (!iommu->external_domain) {
> >>> +	/* Fail if notifier list is empty */
> >>> +	if ((!iommu->external_domain) || (!iommu->notifier.head)) {
> >>>  		ret = -EINVAL;
> >>>  		goto pin_done;
> >>>  	}
> >>> @@ -768,6 +771,50 @@ static unsigned long vfio_pgsize_bitmap(struct vfio_iommu *iommu)
> >>>  	return bitmap;
> >>>  }
> >>>  
> >>> +/*
> >>> + * This function finds pfn in domain->external_addr_space->pfn_list for given
> >>> + * iova range. If pfn exist, notify pfn to registered notifier list. On
> >>> + * receiving notifier callback, vendor driver should invalidate the mapping and
> >>> + * call vfio_unpin_pages() to unpin this pfn. With that vfio_pfn for this pfn
> >>> + * gets removed from rb tree of pfn_list. That re-arranges rb tree, so while
> >>> + * searching for next vfio_pfn in rb tree, start search from first node again.
> >>> + * If any vendor driver doesn't unpin that pfn, vfio_pfn would not get removed
> >>> + * from rb tree and so in next search vfio_pfn would be same as previous
> >>> + * vfio_pfn. In that case, exit from loop.
> >>> + */
> >>> +static void vfio_notifier_call_chain(struct vfio_iommu *iommu,
> >>> +				     struct vfio_iommu_type1_dma_unmap *unmap)
> >>> +{
> >>> +	struct vfio_domain *domain = iommu->external_domain;
> >>> +	struct rb_node *n;
> >>> +	struct vfio_pfn *vpfn = NULL, *prev_vpfn;
> >>> +
> >>> +	do {
> >>> +		prev_vpfn = vpfn;
> >>> +		mutex_lock(&domain->external_addr_space->pfn_list_lock);
> >>> +
> >>> +		n = rb_first(&domain->external_addr_space->pfn_list);
> >>> +
> >>> +		for (; n; n = rb_next(n), vpfn = NULL) {
> >>> +			vpfn = rb_entry(n, struct vfio_pfn, node);
> >>> +
> >>> +			if ((vpfn->iova >= unmap->iova) &&
> >>> +			    (vpfn->iova < unmap->iova + unmap->size))
> >>> +				break;
> >>> +		}
> >>> +
> >>> +		mutex_unlock(&domain->external_addr_space->pfn_list_lock);
> >>> +
> >>> +		/* Notify any listeners about DMA_UNMAP */
> >>> +		if (vpfn)
> >>> +			blocking_notifier_call_chain(&iommu->notifier,
> >>> +						    VFIO_IOMMU_NOTIFY_DMA_UNMAP,
> >>> +						    &vpfn->pfn);    
> >>
> >> Hi Kirti, 
> >>
> >> The information carried by notifier is only a pfn.
> >>
> >> Since your pin/unpin interfaces design, it's the vendor driver who should
> >> guarantee pin/unpin same times. To achieve that, the vendor driver must
> >> cache it's iova->pfn mapping on its side, to avoid pinning a same page
> >> for multiple times.
> >>
> >> With the notifier carrying only a pfn, to find the iova by this pfn,
> >> the vendor driver must *also* keep a reverse-mapping. That's a bit
> >> too much.
> >>
> >> Since the vendor could also suffer from IOMMU-compatible problem,
> >> which means a local cache is always helpful, so I'd like to have the
> >> iova carried to the notifier.
> >>
> >> What'd you say?  
> > 
> > I agree, the pfn is not unique, multiple guest pfns (iovas) might be
> > backed by the same host pfn.  DMA_UNMAP calls are based on iova, the
> > notifier through to the vendor driver must be based on the same.  
> 
> Host pfn should be unique, right?

Let's say a user does a malloc of a single page and does 100 calls to
MAP_DMA populating 100 pages of IOVA space all backed by the same
malloc'd page.  This is valid, I have unit tests that do essentially
this.  Those will all have the same pfn.  The user then does an
UNMAP_DMA to a single one of those IOVA pages.  Did the user unmap
everything matching that pfn?  Of course not, they only unmapped that
one IOVA page.  There is no guarantee of a 1:1 mapping of pfn to IOVA.
UNMAP_DMA works based on IOVA.  Invalidation broadcasts to the vendor
driver MUST therefore also work based on IOVA.  This is not an academic
problem, address space aliases exist in real VMs, imagine a virtual
IOMMU.  Thanks,

Alex