[PATCH] vfio: add a singleton check for vfio_group_pin_pages

[Yan Zhao <yan.y.zhao@intel.com>]

vfio_pin_pages() and vfio_group_pin_pages() are used purely to mark dirty
pages to devices with IOMMU backend as they already have all VM pages
pinned at VM startup.
when there're multiple devices in the vfio group, the dirty pages
marked through pin_pages interface by one device is not useful as the
other devices may access and dirty any VM pages.

So added a check such that only singleton IOMMU groups can pin pages
in vfio_group_pin_pages. for mdevs, there's always only one dev in a
vfio group.
This is a fix to the commit below that added a singleton IOMMU group
check in vfio_pin_pages.

Fixes: 95fc87b44104 (vfio: Selective dirty page tracking if IOMMU backed
device pins pages)

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
---
 drivers/vfio/vfio.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/drivers/vfio/vfio.c b/drivers/vfio/vfio.c
index 5e6e0511b5aa..2f0fa272ebf2 100644
--- a/drivers/vfio/vfio.c
+++ b/drivers/vfio/vfio.c
@@ -2053,6 +2053,9 @@ int vfio_group_pin_pages(struct vfio_group *group,
 	if (!group || !user_iova_pfn || !phys_pfn || !npage)
 		return -EINVAL;
 
+	if (group->dev_counter > 1)
+		return  -EINVAL;
+
 	if (npage > VFIO_PIN_PAGES_MAX_ENTRIES)
 		return -E2BIG;
 
-- 
2.17.1