Messages in this thread Patch in this message | | | Subject | Re: [PATCH] dma-direct: fix DMA_ATTR_NO_KERNEL_MAPPING | From | Walter Wu <> | Date | Mon, 1 Nov 2021 20:07:54 +0800 |
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Hi Robin,
On Mon, 2021-11-01 at 10:29 +0000, Robin Murphy wrote: > 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. >
Yes, this is our previous implementation, but we hope to use kernel memory to fix it.
Thanks for your suggestion.
Walter
> 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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