vfio iommu: Add dma available capability

Commit 49285593 ("vfio/type1: Limit DMA mappings per container")
added the ability to limit the number of memory backed DMA mappings.
However on s390x, when lazy mapping is in use, we use a very large
number of concurrent mappings.  Let's provide the current allowable
number of DMA mappings to userspace via the IOMMU info chain so that
userspace can take appropriate mitigation.
Signed-off-by: Matthew Rosato <mjrosato@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>

drivers/vfio/vfio_iommu_type1.c

@@ -2609,6 +2609,20 @@ static int vfio_iommu_migration_build_caps(struct vfio_iommu *iommu,
 	return vfio_info_add_capability(caps, &cap_mig.header, sizeof(cap_mig));
 }
 
+static int vfio_iommu_dma_avail_build_caps(struct vfio_iommu *iommu,
+					   struct vfio_info_cap *caps)
+{
+	struct vfio_iommu_type1_info_dma_avail cap_dma_avail;
+
+	cap_dma_avail.header.id = VFIO_IOMMU_TYPE1_INFO_DMA_AVAIL;
+	cap_dma_avail.header.version = 1;
+
+	cap_dma_avail.avail = iommu->dma_avail;
+
+	return vfio_info_add_capability(caps, &cap_dma_avail.header,
+					sizeof(cap_dma_avail));
+}
+
 static int vfio_iommu_type1_get_info(struct vfio_iommu *iommu,
 				     unsigned long arg)
 {
@@ -2641,6 +2655,9 @@ static int vfio_iommu_type1_get_info(struct vfio_iommu *iommu,
 
 	ret = vfio_iommu_migration_build_caps(iommu, &caps);
 
+	if (!ret)
+		ret = vfio_iommu_dma_avail_build_caps(iommu, &caps);
+
 	if (!ret)
 		ret = vfio_iommu_iova_build_caps(iommu, &caps);
 

include/uapi/linux/vfio.h

@@ -1039,6 +1039,21 @@ struct vfio_iommu_type1_info_cap_migration {
 	__u64	max_dirty_bitmap_size;		/* in bytes */
 };
 
+/*
+ * The DMA available capability allows to report the current number of
+ * simultaneously outstanding DMA mappings that are allowed.
+ *
+ * The structure below defines version 1 of this capability.
+ *
+ * avail: specifies the current number of outstanding DMA mappings allowed.
+ */
+#define VFIO_IOMMU_TYPE1_INFO_DMA_AVAIL 3
+
+struct vfio_iommu_type1_info_dma_avail {
+	struct	vfio_info_cap_header header;
+	__u32	avail;
+};
+
 #define VFIO_IOMMU_GET_INFO _IO(VFIO_TYPE, VFIO_BASE + 12)
 
 /**