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Re: [PATCH RFC] xen/swiotlb: avoid arch_sync_dma_* on per-device DMA memory
- To: "Peng Fan (OSS)" <peng.fan@xxxxxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Oleksandr Tyshchenko <oleksandr_tyshchenko@xxxxxxxx>
- From: Jürgen Groß <jgross@xxxxxxxx>
- Date: Mon, 20 Apr 2026 14:19:34 +0200
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- Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, iommu@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, Peng Fan <peng.fan@xxxxxxx>
- Delivery-date: Mon, 20 Apr 2026 12:19:46 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 15.04.26 17:08, Peng Fan (OSS) wrote:
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;
+}
+
I don't think this will compile on x86.
Juergen
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