On Tue, 30 Dec 2025 05:15:46 -0500
"Michael S. Tsirkin" <[email protected]> wrote:
> 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
> Signed-off-by: Michael S. Tsirkin <[email protected]>
> ---
> 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)
> {
