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

On Fri, Apr 23, 2021 at 7:34 PM Steven Price [off-list ref] wrote:

On 22/04/2021 09:14, Claire Chang wrote:

quoted

Add the initialization function to create restricted DMA pools from
matching reserved-memory nodes.

Signed-off-by: Claire Chang <redacted>
---
  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.

Thanks for pointing this out.

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.

debugfs creation should move into the if (!mem) {...} above to avoid
duplication.
I think having a separated struct dentry pointer for the default
debugfs should be enough?

if (!debugfs)
    debugfs = debugfs_create_dir("swiotlb", NULL);
swiotlb_create_debugfs(mem, rmem->name, debugfs);

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