Messages in this thread | | | Subject | Re: [PATCH 1/2] dma-mapping: support setting memory uncached in place | From | Robin Murphy <> | Date | Thu, 20 Feb 2020 17:21:35 +0000 |
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On 20/02/2020 5:01 pm, Christoph Hellwig wrote: > We currently only support remapping memory as uncached through vmap > or a magic uncached segment provided by some architectures. But there > is a simpler and much better way available on some architectures where > we can just remap the memory in place. The advantages are: > > 1) no aliasing is possible, which prevents speculating into the > cached alias > 2) there is no need to allocate new ptes and thus no need for a special > pre-allocated pool of memory that can be used with GFP_ATOMIC DMA > allocations > > The downside is that architectures must provide a way to set arbitrary > pages uncached in the kernel mapping, which might not be possible on > architecture that have a special implicit kernel mapping, and requires > splitting of huge page kernel mappings where they exist. > > Signed-off-by: Christoph Hellwig <hch@lst.de> > --- > include/linux/dma-noncoherent.h | 3 +++ > kernel/dma/Kconfig | 8 ++++++++ > kernel/dma/direct.c | 28 ++++++++++++++++++---------- > 3 files changed, 29 insertions(+), 10 deletions(-) > > diff --git a/include/linux/dma-noncoherent.h b/include/linux/dma-noncoherent.h > index ca9b5770caee..0820ec58f119 100644 > --- a/include/linux/dma-noncoherent.h > +++ b/include/linux/dma-noncoherent.h > @@ -111,4 +111,7 @@ static inline void arch_dma_prep_coherent(struct page *page, size_t size) > void *uncached_kernel_address(void *addr); > void *cached_kernel_address(void *addr); > > +int arch_dma_set_uncached(void *cpu_addr, size_t size); > +void arch_dma_clear_uncached(void *cpu_addr, size_t size); > + > #endif /* _LINUX_DMA_NONCOHERENT_H */ > diff --git a/kernel/dma/Kconfig b/kernel/dma/Kconfig > index 4c103a24e380..7bc0b77f1243 100644 > --- a/kernel/dma/Kconfig > +++ b/kernel/dma/Kconfig > @@ -83,6 +83,14 @@ config DMA_DIRECT_REMAP > bool > select DMA_REMAP > > +# > +# Should be selected if the architecture can remap memory from the page > +# allocator and CMA as uncached and provides the arch_dma_set_uncached and > +# arch_dma_clear_uncached helpers > +# > +config ARCH_HAS_DMA_SET_UNCACHED > + bool > + > config DMA_CMA > bool "DMA Contiguous Memory Allocator" > depends on HAVE_DMA_CONTIGUOUS && CMA > diff --git a/kernel/dma/direct.c b/kernel/dma/direct.c > index 6af7ae83c4ad..73fe65a4cbc0 100644 > --- a/kernel/dma/direct.c > +++ b/kernel/dma/direct.c > @@ -169,11 +169,8 @@ void *dma_direct_alloc_pages(struct device *dev, size_t size, > ret = dma_common_contiguous_remap(page, PAGE_ALIGN(size), > dma_pgprot(dev, PAGE_KERNEL, attrs), > __builtin_return_address(0)); > - if (!ret) { > - dma_free_contiguous(dev, page, size); > - return ret; > - } > - > + if (!ret) > + goto out_free_pages; > memset(ret, 0, size); > goto done; > } > @@ -186,8 +183,7 @@ void *dma_direct_alloc_pages(struct device *dev, size_t size, > * so log an error and fail. > */ > dev_info(dev, "Rejecting highmem page from CMA.\n"); > - dma_free_contiguous(dev, page, size); > - return NULL; > + goto out_free_pages; > } > > ret = page_address(page); > @@ -196,10 +192,15 @@ void *dma_direct_alloc_pages(struct device *dev, size_t size, > > memset(ret, 0, size); > > - if (IS_ENABLED(CONFIG_ARCH_HAS_UNCACHED_SEGMENT) && > - dma_alloc_need_uncached(dev, attrs)) { > + if (dma_alloc_need_uncached(dev, attrs)) { > arch_dma_prep_coherent(page, size); > - ret = uncached_kernel_address(ret); > + > + if (IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED)) { > + if (!arch_dma_set_uncached(ret, size)) > + goto out_free_pages; > + } else if (IS_ENABLED(CONFIG_ARCH_HAS_UNCACHED_SEGMENT)) { > + ret = uncached_kernel_address(ret);
Hmm, would we actually need to keep ARCH_HAS_UNCACHED_SEGMENT? If arch_dma_set_uncached() returned void*/ERR_PTR instead, then it could work for both cases (with arch_dma_clear_uncached() being a no-op for segments).
Robin.
> + } > } > done: > if (force_dma_unencrypted(dev)) > @@ -207,6 +208,9 @@ void *dma_direct_alloc_pages(struct device *dev, size_t size, > else > *dma_handle = phys_to_dma(dev, page_to_phys(page)); > return ret; > +out_free_pages: > + dma_free_contiguous(dev, page, size); > + return NULL; > } > > void dma_direct_free_pages(struct device *dev, size_t size, void *cpu_addr, > @@ -230,6 +234,8 @@ void dma_direct_free_pages(struct device *dev, size_t size, void *cpu_addr, > > if (IS_ENABLED(CONFIG_DMA_REMAP) && is_vmalloc_addr(cpu_addr)) > vunmap(cpu_addr); > + else if (IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED)) > + arch_dma_clear_uncached(cpu_addr, size); > > dma_free_contiguous(dev, dma_direct_to_page(dev, dma_addr), size); > } > @@ -238,6 +244,7 @@ void *dma_direct_alloc(struct device *dev, size_t size, > dma_addr_t *dma_handle, gfp_t gfp, unsigned long attrs) > { > if (!IS_ENABLED(CONFIG_ARCH_HAS_UNCACHED_SEGMENT) && > + !IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED) && > !IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) && > dma_alloc_need_uncached(dev, attrs)) > return arch_dma_alloc(dev, size, dma_handle, gfp, attrs); > @@ -248,6 +255,7 @@ void dma_direct_free(struct device *dev, size_t size, > void *cpu_addr, dma_addr_t dma_addr, unsigned long attrs) > { > if (!IS_ENABLED(CONFIG_ARCH_HAS_UNCACHED_SEGMENT) && > + !IS_ENABLED(CONFIG_ARCH_HAS_DMA_SET_UNCACHED) && > !IS_ENABLED(CONFIG_DMA_DIRECT_REMAP) && > dma_alloc_need_uncached(dev, attrs)) > arch_dma_free(dev, size, cpu_addr, dma_addr, attrs); >
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