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    SubjectRe: [PATCH] dma-direct: fix DMA_ATTR_NO_KERNEL_MAPPING
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    Hi Ard,

    On Mon, 2021-11-01 at 09:34 +0100, Ard Biesheuvel wrote:
    > On Mon, 1 Nov 2021 at 04:17, Walter Wu <walter-zh.wu@mediatek.com>
    > 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.
    > >
    > > 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.
    > >
    > > 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);
    >
    > This only works if the memory is mapped at page granularity in the
    > linear region, and you cannot rely on that. Many architectures prefer
    > block mappings for the linear region, and arm64 will only use page
    > mappings if rodata=full is set (which is set by default but can be
    > overridden on the kernel command line)
    >

    We mainly want to solve arm64 arch. RODATA_FULL_DEFAULT_ENABLED should
    be the arm64 config. If we use CONFIG_RODATA_FULL_DEFAULT_ENABLED to
    check whether it call set_memory_valid(). It should avoid other
    architectures. Do you think this method is work?

    Thanks for your explaination and suggestion.

    Walter

    >
    > > /* 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;
    > > --
    > > 2.18.0
    > >
    > >
    > > _______________________________________________
    > > linux-arm-kernel mailing list
    > > linux-arm-kernel@lists.infradead.org
    > >
    https://urldefense.com/v3/__http://lists.infradead.org/mailman/listinfo/linux-arm-kernel__;!!CTRNKA9wMg0ARbw!16dLCjnvtRkaRLeCO9AQ7Fund5XL0FicZmeVaU3-WkFymr-0lbITfzwrvoJpiHlqnqIu-g$
    > >

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