On Wed, Dec 31, 2025 at 03:01:59PM +0100, Petr Tesarik wrote:

On Tue, 30 Dec 2025 05:15:46 -0500
"Michael S. Tsirkin" [off-list ref] wrote:

quoted

When a structure contains a buffer that DMA writes to alongside fields
that the CPU writes to, cache line sharing between the DMA buffer and
CPU-written fields can cause data corruption on non-cache-coherent
platforms.

Add __dma_from_device_aligned_begin/__dma_from_device_aligned_end
annotations to ensure proper alignment to prevent this:

struct my_device {
	spinlock_t lock1;
	__dma_from_device_aligned_begin char dma_buffer1[16];
	char dma_buffer2[16];
	__dma_from_device_aligned_end spinlock_t lock2;
};

When the DMA buffer is the last field in the structure, just
__dma_from_device_aligned_begin is enough - the compiler's struct
padding protects the tail:

struct my_device {
	spinlock_t lock;
	struct mutex mlock;
	__dma_from_device_aligned_begin char dma_buffer1[16];
	char dma_buffer2[16];
};

This works, but it's a bit hard to read. Can we reuse the
__cacheline_group_{begin, end}() macros from <linux/cache.h>?
Something like this:

#define __dma_from_device_group_begin(GROUP)			\
	__cacheline_group_begin(GROUP)				\
	____dma_from_device_aligned
#define __dma_from_device_group_end(GROUP)			\
	__cacheline_group_end(GROUP)				\
	____dma_from_device_aligned

And used like this (the "rxbuf" group id was chosen arbitrarily):

struct my_device {
	spinlock_t lock1;
	__dma_from_device_group_begin(rxbuf);
	char dma_buffer1[16];
	char dma_buffer2[16];
	__dma_from_device_group_end(rxbuf);
	spinlock_t lock2;
};

Petr T

Oh, that's a clever idea!

Will do! And GROUP is optional if there's only one group in a structure.

quoted

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
---
 include/linux/dma-mapping.h | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
index aa36a0d1d9df..47b7de3786a1 100644
--- a/include/linux/dma-mapping.h
+++ b/include/linux/dma-mapping.h

@@ -703,6 +703,16 @@ static inline int dma_get_cache_alignment(void)
 }
 #endif
 
+#ifdef ARCH_HAS_DMA_MINALIGN
+#define ____dma_from_device_aligned __aligned(ARCH_DMA_MINALIGN)
+#else
+#define ____dma_from_device_aligned
+#endif
+/* Apply to the 1st field of the DMA buffer */
+#define __dma_from_device_aligned_begin ____dma_from_device_aligned
+/* Apply to the 1st field beyond the DMA buffer */
+#define __dma_from_device_aligned_end ____dma_from_device_aligned
+
 static inline void *dmam_alloc_coherent(struct device *dev, size_t size,
 		dma_addr_t *dma_handle, gfp_t gfp)
 {