linux-kernel - Re: [PATCH RFC 01/13] dma-mapping: add __dma_from_device_align

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Message-ID: <20251231150159.1779b585@mordecai>
Date: Wed, 31 Dec 2025 15:01:59 +0100
From: Petr Tesarik <ptesarik@...e.com>
To: "Michael S. Tsirkin" <mst@...hat.com>
Cc: linux-kernel@...r.kernel.org, Cong Wang <xiyou.wangcong@...il.com>,
 Jonathan Corbet <corbet@....net>, Olivia Mackall <olivia@...enic.com>,
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 <James.Bottomley@...senpartnership.com>, "Martin K. Petersen"
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Subject: Re: [PATCH RFC 01/13] dma-mapping: add
 __dma_from_device_align_begin/end

On Tue, 30 Dec 2025 05:15:46 -0500
"Michael S. Tsirkin" <mst@...hat.com> 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 <mst@...hat.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)
>  {