Re: [PATCH] dma-direct: fix DMA_ATTR_NO_KERNEL_MAPPING

[Robin Murphy]

On 2021-11-01 03:15, Walter Wu wrote:
> DMA_ATTR_NO_KERNEL_MAPPING is to avoid creating a kernel mapping
> for the allocated buffer, but current implementation is that
> PTE of allocated buffer in kernel page table is valid. So we
> should set invalid for PTE of allocate buffer so that there are
> no kernel mapping for the allocated buffer.

No, the semantic of NO_KERNEL_MAPPING is an indication that the *caller* 
does not need a mapping, such that the DMA API implementation may choose 
to optimise for that internally. It has never given any guarantee of any 
particular behaviour - like most attributes it is only a hint.

> In some cases, we don't hope the allocated buffer to be read
> by cpu or speculative execution, so we use DMA_ATTR_NO_KERNEL_MAPPING
> to get no kernel mapping in order to achieve this goal.

If it's important that no CPU accesses to this memory can happen, then I 
think the only way to absolutely guarantee that is to exclude it from 
the kernel's memory map in the first place, e.g. as a DT reserved-memory 
region with the "no-map" property.

Robin.

> Signed-off-by: Walter Wu <walter-zh.wu@xxxxxxxxxxxx>
> Cc: Christoph Hellwig <hch@xxxxxx>
> Cc: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>
> Cc: Robin Murphy <robin.murphy@xxxxxxx>
> Cc: Matthias Brugger <matthias.bgg@xxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> ---
>   kernel/dma/direct.c | 8 ++++++++
>   1 file changed, 8 insertions(+)
>
> diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c
> index 4c6c5e0635e3..aa10b4c5d762 100644
> --- a/kernel/dma/direct.c
> +++ b/kernel/dma/direct.c
> @@ -13,6 +13,7 @@
>   #include <linux/vmalloc.h>
>   #include <linux/set_memory.h>
>   #include <linux/slab.h>
> +#include <asm/cacheflush.h>
>   #include "direct.h"
>   
>   /*
> @@ -169,6 +170,9 @@ void *dma_direct_alloc(struct device *dev, size_t size,
>   		if (!PageHighMem(page))
>   			arch_dma_prep_coherent(page, size);
>   		*dma_handle = phys_to_dma_direct(dev, page_to_phys(page));
> +		/* remove kernel mapping for pages */
> +		set_memory_valid((unsigned long)phys_to_virt(dma_to_phys(dev, *dma_handle)),
> +				size >> PAGE_SHIFT, 0);
>   		/* return the page pointer as the opaque cookie */
>   		return page;
>   	}
> @@ -278,6 +282,10 @@ void dma_direct_free(struct device *dev, size_t size,
>   
>   	if ((attrs & DMA_ATTR_NO_KERNEL_MAPPING) &&
>   	    !force_dma_unencrypted(dev) && !is_swiotlb_for_alloc(dev)) {
> +		size = PAGE_ALIGN(size);
> +		/* create kernel mapping for pages */
> +		set_memory_valid((unsigned long)phys_to_virt(dma_to_phys(dev, dma_addr)),
> +				size >> PAGE_SHIFT, 1);
>   		/* cpu_addr is a struct page cookie, not a kernel address */
>   		dma_free_contiguous(dev, cpu_addr, size);
>   		return;