dma-iommu: use static-key to minimize the impact in the fast-path

Let's move out the is_kdump_kernel() check from iommu_dma_deferred_attach()
to iommu_dma_init(), and use the static-key in the fast-path to minimize
the impact in the normal case.
Co-developed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Lianbo Jiang <lijiang@redhat.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/20210126115337.20068-2-lijiang@redhat.comSigned-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/dma-iommu.c

@@ -51,6 +51,8 @@ struct iommu_dma_cookie {
 	struct iommu_domain		*fq_domain;
 };
 
+static DEFINE_STATIC_KEY_FALSE(iommu_deferred_attach_enabled);
+
 void iommu_dma_free_cpu_cached_iovas(unsigned int cpu,
 		struct iommu_domain *domain)
 {
@@ -383,9 +385,6 @@ static int iommu_dma_deferred_attach(struct device *dev,
 {
 	const struct iommu_ops *ops = domain->ops;
 
-	if (!is_kdump_kernel())
-		return 0;
-
 	if (unlikely(ops->is_attach_deferred &&
 			ops->is_attach_deferred(domain, dev)))
 		return iommu_attach_device(domain, dev);
@@ -535,7 +534,8 @@ static dma_addr_t __iommu_dma_map(struct device *dev, phys_addr_t phys,
 	size_t iova_off = iova_offset(iovad, phys);
 	dma_addr_t iova;
 
-	if (unlikely(iommu_dma_deferred_attach(dev, domain)))
+	if (static_branch_unlikely(&iommu_deferred_attach_enabled) &&
+	    iommu_dma_deferred_attach(dev, domain))
 		return DMA_MAPPING_ERROR;
 
 	size = iova_align(iovad, size + iova_off);
@@ -693,7 +693,8 @@ static void *iommu_dma_alloc_remap(struct device *dev, size_t size,
 
 	*dma_handle = DMA_MAPPING_ERROR;
 
-	if (unlikely(iommu_dma_deferred_attach(dev, domain)))
+	if (static_branch_unlikely(&iommu_deferred_attach_enabled) &&
+	    iommu_dma_deferred_attach(dev, domain))
 		return NULL;
 
 	min_size = alloc_sizes & -alloc_sizes;
@@ -976,7 +977,8 @@ static int iommu_dma_map_sg(struct device *dev, struct scatterlist *sg,
 	unsigned long mask = dma_get_seg_boundary(dev);
 	int i;
 
-	if (unlikely(iommu_dma_deferred_attach(dev, domain)))
+	if (static_branch_unlikely(&iommu_deferred_attach_enabled) &&
+	    iommu_dma_deferred_attach(dev, domain))
 		return 0;
 
 	if (!(attrs & DMA_ATTR_SKIP_CPU_SYNC))
@@ -1424,6 +1426,9 @@ void iommu_dma_compose_msi_msg(struct msi_desc *desc,
 
 static int iommu_dma_init(void)
 {
+	if (is_kdump_kernel())
+		static_branch_enable(&iommu_deferred_attach_enabled);
+
 	return iova_cache_get();
 }
 arch_initcall(iommu_dma_init);