Messages in this thread | | | Date | Mon, 1 Nov 2021 10:29:19 +0000 | Subject | Re: [PATCH] dma-direct: fix DMA_ATTR_NO_KERNEL_MAPPING | From | Robin Murphy <> |
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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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