[PATCH RFC] xen/swiotlb: avoid arch_sync_dma_* on per-device DMA memory

["Peng Fan (OSS)" <peng.fan@xxxxxxxxxxx>]

From: Peng Fan <peng.fan@xxxxxxx>

On ARM64, arch_sync_dma_for_{cpu,device}() assumes that the
physical address passed in refers to normal RAM that is part of the
kernel linear(direct) mapping, as it unconditionally derives a CPU
virtual address via phys_to_virt().

With Xen swiotlb, devices may use per-device coherent DMA memory,
such as reserved-memory regions described by 'shared-dma-pool',
which are assigned to dev->dma_mem. These regions may be marked
no-map in DT and therefore are not part of the kernel linear map.
In such cases, pfn_valid() still returns true, but phys_to_virt()
is not valid and cache maintenance via arch_sync_dma_* will fault.

Prevent this by excluding devices with a private DMA memory pool
(dev->dma_mem) from the arch_sync_dma_* fast path, and always
fall back to xen_dma_sync_* for those devices to avoid invalid
phys_to_virt() conversions for no-map DMA memory while preserving the
existing fast path for normal, linear-mapped RAM.

Signed-off-by: Peng Fan <peng.fan@xxxxxxx>
---
 drivers/xen/swiotlb-xen.c | 17 +++++++++++++----
 1 file changed, 13 insertions(+), 4 deletions(-)

diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
index 
2cbf2b588f5b20cfbf9e83a8339dc22092c9559a..b1445df99d9a8f1d18a83b8c413bada6e5579209
 100644
--- a/drivers/xen/swiotlb-xen.c
+++ b/drivers/xen/swiotlb-xen.c
@@ -195,6 +195,11 @@ xen_swiotlb_free_coherent(struct device *dev, size_t size, 
void *vaddr,
 }
 #endif /* CONFIG_X86 */
 
+static inline bool dev_has_private_dma_pool(struct device *dev)
+{
+       return dev && dev->dma_mem;
+}
+
 /*
  * Map a single buffer of the indicated size for DMA in streaming mode.  The
  * physical address to use is returned.
@@ -262,7 +267,8 @@ static dma_addr_t xen_swiotlb_map_phys(struct device *dev, 
phys_addr_t phys,
 
 done:
        if (!dev_is_dma_coherent(dev) && !(attrs & DMA_ATTR_SKIP_CPU_SYNC)) {
-               if (pfn_valid(PFN_DOWN(dma_to_phys(dev, dev_addr)))) {
+               if (pfn_valid(PFN_DOWN(dma_to_phys(dev, dev_addr))) &&
+                   !dev_has_private_dma_pool(dev)) {
                        arch_sync_dma_for_device(phys, size, dir);
                        arch_sync_dma_flush();
                } else {
@@ -289,7 +295,8 @@ static void xen_swiotlb_unmap_phys(struct device *hwdev, 
dma_addr_t dev_addr,
        BUG_ON(dir == DMA_NONE);
 
        if (!dev_is_dma_coherent(hwdev) && !(attrs & DMA_ATTR_SKIP_CPU_SYNC)) {
-               if (pfn_valid(PFN_DOWN(dma_to_phys(hwdev, dev_addr)))) {
+               if (pfn_valid(PFN_DOWN(dma_to_phys(hwdev, dev_addr))) &&
+                   !dev_has_private_dma_pool(hwdev)) {
                        arch_sync_dma_for_cpu(paddr, size, dir);
                        arch_sync_dma_flush();
                } else {
@@ -312,7 +319,8 @@ xen_swiotlb_sync_single_for_cpu(struct device *dev, 
dma_addr_t dma_addr,
        struct io_tlb_pool *pool;
 
        if (!dev_is_dma_coherent(dev)) {
-               if (pfn_valid(PFN_DOWN(dma_to_phys(dev, dma_addr)))) {
+               if (pfn_valid(PFN_DOWN(dma_to_phys(dev, dma_addr))) &&
+                   !dev_has_private_dma_pool(dev)) {
                        arch_sync_dma_for_cpu(paddr, size, dir);
                        arch_sync_dma_flush();
                } else {
@@ -337,7 +345,8 @@ xen_swiotlb_sync_single_for_device(struct device *dev, 
dma_addr_t dma_addr,
                __swiotlb_sync_single_for_device(dev, paddr, size, dir, pool);
 
        if (!dev_is_dma_coherent(dev)) {
-               if (pfn_valid(PFN_DOWN(dma_to_phys(dev, dma_addr)))) {
+               if (pfn_valid(PFN_DOWN(dma_to_phys(dev, dma_addr))) &&
+                   !dev_has_private_dma_pool(dev)) {
                        arch_sync_dma_for_device(paddr, size, dir);
                        arch_sync_dma_flush();
                } else {

---
base-commit: 66672af7a095d89f082c5327f3b15bc2f93d558e
change-id: 20260415-xen-swiotlb-34a198b6c1d6

Best regards,
-- 
Peng Fan <peng.fan@xxxxxxx>