Subject: Re: [PATCH RFC 01/13] dma-mapping: add __dma_from_device_align_begin/end

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 31 Dec 2025 09:40:34 -0500

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" <[email-masked]> 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

Oh, that's a clever idea!

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

> > Signed-off-by: Michael S. Tsirkin <[email-masked]>
> > ---
> >  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)
> >  {