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SubjectRe: [PATCH] dma-direct: fix DMA_ATTR_NO_KERNEL_MAPPING
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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@mediatek.com>
> Cc: Christoph Hellwig <hch@lst.de>
> Cc: Marek Szyprowski <m.szyprowski@samsung.com>
> Cc: Robin Murphy <robin.murphy@arm.com>
> Cc: Matthias Brugger <matthias.bgg@gmail.com>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> ---
> 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;
>

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