Re: [PATCH v5 05/16] swiotlb: Add restricted DMA pool initialization

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On 22/04/2021 09:14, Claire Chang wrote:
Add the initialization function to create restricted DMA pools from
matching reserved-memory nodes.

Signed-off-by: Claire Chang <tientzu@xxxxxxxxxxxx>
---
  include/linux/device.h  |  4 +++
  include/linux/swiotlb.h |  3 +-
  kernel/dma/swiotlb.c    | 80 +++++++++++++++++++++++++++++++++++++++++
  3 files changed, 86 insertions(+), 1 deletion(-)

diff --git a/include/linux/device.h b/include/linux/device.h
index 38a2071cf776..4987608ea4ff 100644
--- a/include/linux/device.h
+++ b/include/linux/device.h
@@ -416,6 +416,7 @@ struct dev_links_info {
   * @dma_pools:	Dma pools (if dma'ble device).
   * @dma_mem:	Internal for coherent mem override.
   * @cma_area:	Contiguous memory area for dma allocations
+ * @dma_io_tlb_mem: Internal for swiotlb io_tlb_mem override.
   * @archdata:	For arch-specific additions.
   * @of_node:	Associated device tree node.
   * @fwnode:	Associated device node supplied by platform firmware.
@@ -521,6 +522,9 @@ struct device {
  #ifdef CONFIG_DMA_CMA
  	struct cma *cma_area;		/* contiguous memory area for dma
  					   allocations */
+#endif
+#ifdef CONFIG_DMA_RESTRICTED_POOL
+	struct io_tlb_mem *dma_io_tlb_mem;
  #endif
  	/* arch specific additions */
  	struct dev_archdata	archdata;
diff --git a/include/linux/swiotlb.h b/include/linux/swiotlb.h
index 216854a5e513..03ad6e3b4056 100644
--- a/include/linux/swiotlb.h
+++ b/include/linux/swiotlb.h
@@ -72,7 +72,8 @@ extern enum swiotlb_force swiotlb_force;
   *		range check to see if the memory was in fact allocated by this
   *		API.
   * @nslabs:	The number of IO TLB blocks (in groups of 64) between @start and
- *		@end. This is command line adjustable via setup_io_tlb_npages.
+ *		@end. For default swiotlb, this is command line adjustable via
+ *		setup_io_tlb_npages.
   * @used:	The number of used IO TLB block.
   * @list:	The free list describing the number of free entries available
   *		from each index.
diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
index 57a9adb920bf..ffbb8724e06c 100644
--- a/kernel/dma/swiotlb.c
+++ b/kernel/dma/swiotlb.c
@@ -39,6 +39,13 @@
  #ifdef CONFIG_DEBUG_FS
  #include <linux/debugfs.h>
  #endif
+#ifdef CONFIG_DMA_RESTRICTED_POOL
+#include <linux/io.h>
+#include <linux/of.h>
+#include <linux/of_fdt.h>
+#include <linux/of_reserved_mem.h>
+#include <linux/slab.h>
+#endif
#include <asm/io.h>
  #include <asm/dma.h>
@@ -681,3 +688,76 @@ static int __init swiotlb_create_default_debugfs(void)
  late_initcall(swiotlb_create_default_debugfs);
#endif
+
+#ifdef CONFIG_DMA_RESTRICTED_POOL
+static int rmem_swiotlb_device_init(struct reserved_mem *rmem,
+				    struct device *dev)
+{
+	struct io_tlb_mem *mem = rmem->priv;
+	unsigned long nslabs = rmem->size >> IO_TLB_SHIFT;
+
+	if (dev->dma_io_tlb_mem)
+		return 0;
+
+	/* Since multiple devices can share the same pool, the private data,
+	 * io_tlb_mem struct, will be initialized by the first device attached
+	 * to it.
+	 */
+	if (!mem) {
+		mem = kzalloc(struct_size(mem, slots, nslabs), GFP_KERNEL);
+		if (!mem)
+			return -ENOMEM;
+#ifdef CONFIG_ARM
+		if (!PageHighMem(pfn_to_page(PHYS_PFN(rmem->base)))) {
+			kfree(mem);
+			return -EINVAL;
+		}
+#endif /* CONFIG_ARM */
+		swiotlb_init_io_tlb_mem(mem, rmem->base, nslabs, false);
+
+		rmem->priv = mem;
+	}
+
+#ifdef CONFIG_DEBUG_FS
+	if (!io_tlb_default_mem->debugfs)
+		io_tlb_default_mem->debugfs =
+			debugfs_create_dir("swiotlb", NULL);

At this point it's possible for io_tlb_default_mem to be NULL, leading to a splat.

But even then if it's not and we have the situation where debugfs==NULL then the debugfs_create_dir() here will cause a subsequent attempt in swiotlb_create_debugfs() to fail (directory already exists) leading to mem->debugfs being assigned an error value. I suspect the creation of the debugfs directory needs to be separated from io_tlb_default_mem being set.

Other than that I gave this series a go with our prototype of Arm's Confidential Computer Architecture[1] - since the majority of the guest's memory is protected from the host the restricted DMA pool allows (only) a small area to be shared with the host.

After fixing (well hacking round) the above it all seems to be working fine with virtio drivers.

Thanks,

Steve

[1] https://www.arm.com/why-arm/architecture/security-features/arm-confidential-compute-architecture



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