netdev - Re: [PATCH v2 03/15] dma-mapping: add DMA_ATTR_CPU_CACHE

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Message-ID: <20260105105018.732dd009@mordecai>
Date: Mon, 5 Jan 2026 10:50:18 +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>,
 Herbert Xu <herbert@...dor.apana.org.au>, Jason Wang <jasowang@...hat.com>,
 Paolo Bonzini <pbonzini@...hat.com>, Stefan Hajnoczi <stefanha@...hat.com>,
 Eugenio Pérez <eperezma@...hat.com>, "James E.J. Bottomley"
 <James.Bottomley@...senpartnership.com>, "Martin K. Petersen"
 <martin.petersen@...cle.com>, Gerd Hoffmann <kraxel@...hat.com>, Xuan Zhuo
 <xuanzhuo@...ux.alibaba.com>, Marek Szyprowski <m.szyprowski@...sung.com>,
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Subject: Re: [PATCH v2 03/15] dma-mapping: add DMA_ATTR_CPU_CACHE_CLEAN

On Mon, 5 Jan 2026 03:23:01 -0500
"Michael S. Tsirkin" <mst@...hat.com> wrote:

> When multiple small DMA_FROM_DEVICE or DMA_BIDIRECTIONAL buffers share a
> cacheline, and DMA_API_DEBUG is enabled, we get this warning:
> 	cacheline tracking EEXIST, overlapping mappings aren't supported.
> 
> This is because when one of the mappings is removed, while another one
> is active, CPU might write into the buffer.
> 
> Add an attribute for the driver to promise not to do this, making the
> overlapping safe, and suppressing the warning.
> 
> Signed-off-by: Michael S. Tsirkin <mst@...hat.com>

LGTM. I'm not formally a reviewer, but FWIW:

Reviewed-by: Petr Tesarik <ptesarik@...e.com>

> ---
>  include/linux/dma-mapping.h | 7 +++++++
>  kernel/dma/debug.c          | 3 ++-
>  2 files changed, 9 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/dma-mapping.h b/include/linux/dma-mapping.h
> index 29ad2ce700f0..29973baa0581 100644
> --- a/include/linux/dma-mapping.h
> +++ b/include/linux/dma-mapping.h
> @@ -79,6 +79,13 @@
>   */
>  #define DMA_ATTR_MMIO		(1UL << 10)
>  
> +/*
> + * DMA_ATTR_CPU_CACHE_CLEAN: Indicates the CPU will not dirty any cacheline
> + * overlapping this buffer while it is mapped for DMA. All mappings sharing
> + * a cacheline must have this attribute for this to be considered safe.
> + */
> +#define DMA_ATTR_CPU_CACHE_CLEAN	(1UL << 11)
> +
>  /*
>   * A dma_addr_t can hold any valid DMA or bus address for the platform.  It can
>   * be given to a device to use as a DMA source or target.  It is specific to a
> diff --git a/kernel/dma/debug.c b/kernel/dma/debug.c
> index 138ede653de4..7e66d863d573 100644
> --- a/kernel/dma/debug.c
> +++ b/kernel/dma/debug.c
> @@ -595,7 +595,8 @@ static void add_dma_entry(struct dma_debug_entry *entry, unsigned long attrs)
>  	if (rc == -ENOMEM) {
>  		pr_err_once("cacheline tracking ENOMEM, dma-debug disabled\n");
>  		global_disable = true;
> -	} else if (rc == -EEXIST && !(attrs & DMA_ATTR_SKIP_CPU_SYNC) &&
> +	} else if (rc == -EEXIST &&
> +		   !(attrs & (DMA_ATTR_SKIP_CPU_SYNC | DMA_ATTR_CPU_CACHE_CLEAN)) &&
>  		   !(IS_ENABLED(CONFIG_DMA_BOUNCE_UNALIGNED_KMALLOC) &&
>  		     is_swiotlb_active(entry->dev))) {
>  		err_printk(entry->dev, entry,