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Re: [PATCH] dma-buf: system_heap: Optimize sg_table-to-pages conversion in vmap

by Christian König

On 4/6/26 23:49, Barry Song (Xiaomi) wrote: > From: Xueyuan Chen <Xueyuan.chen21(a)gmail.com> > > Replace the heavy for_each_sgtable_page() iterator in system_heap_do_vmap() > with a more efficient nested loop approach. > > Instead of iterating page by page, we now iterate through the scatterlist > entries via for_each_sgtable_sg(). Because pages within a single sg entry > are physically contiguous, we can populate the page array with a in an > inner loop using simple pointer math. This save a lot of time. > > The WARN_ON check is also pulled out of the loop to save branch > instructions. > > Performance results mapping a 2GB buffer on Radxa O6: > - Before: ~1440000 ns > - After: ~232000 ns > (~84% reduction in iteration time, or ~6.2x faster) Well real question is why do you care about the vmap performance? That should basically only be used for fbdev emulation (except for VMGFX) and we absolutely don't care about performance there. Regards, Christian. > > Cc: Sumit Semwal <sumit.semwal(a)linaro.org> > Cc: Benjamin Gaignard <benjamin.gaignard(a)collabora.com> > Cc: Brian Starkey <Brian.Starkey(a)arm.com> > Cc: John Stultz <jstultz(a)google.com> > Cc: T.J. Mercier <tjmercier(a)google.com> > Cc: Christian König <christian.koenig(a)amd.com> > Signed-off-by: Xueyuan Chen <Xueyuan.chen21(a)gmail.com> > Signed-off-by: Barry Song (Xiaomi) <baohua(a)kernel.org> > --- > drivers/dma-buf/heaps/system_heap.c | 13 +++++++++---- > 1 file changed, 9 insertions(+), 4 deletions(-) > > diff --git a/drivers/dma-buf/heaps/system_heap.c b/drivers/dma-buf/heaps/system_heap.c > index b3650d8fd651..769f01f0cc96 100644 > --- a/drivers/dma-buf/heaps/system_heap.c > +++ b/drivers/dma-buf/heaps/system_heap.c > @@ -224,16 +224,21 @@ static void *system_heap_do_vmap(struct system_heap_buffer *buffer) > int npages = PAGE_ALIGN(buffer->len) / PAGE_SIZE; > struct page **pages = vmalloc(sizeof(struct page *) * npages); > struct page **tmp = pages; > - struct sg_page_iter piter; > void *vaddr; > + u32 i, j, count; > + struct page *base_page; > + struct scatterlist *sg; > > if (!pages) > return ERR_PTR(-ENOMEM); > > - for_each_sgtable_page(table, &piter, 0) { > - WARN_ON(tmp - pages >= npages); > - *tmp++ = sg_page_iter_page(&piter); > + for_each_sgtable_sg(table, sg, i) { > + base_page = sg_page(sg); > + count = sg->length >> PAGE_SHIFT; > + for (j = 0; j < count; j++) > + *tmp++ = base_page + j; > } > + WARN_ON(tmp - pages != npages); > > vaddr = vmap(pages, npages, VM_MAP, PAGE_KERNEL); > vfree(pages);

13 minutes

Re: [PATCH] dma-buf: heaps: system: document system_cc_shared heap

by T.J. Mercier

On Thu, Apr 2, 2026 at 7:11 AM Jiri Pirko <jiri(a)resnulli.us> wrote: > > From: Jiri Pirko <jiri(a)nvidia.com> > > Document the system_cc_shared dma-buf heap that was introduced > recently. Describe its purpose, availability conditions and > relation to confidential computing VMs. > > Signed-off-by: Jiri Pirko <jiri(a)nvidia.com> > --- > Documentation/userspace-api/dma-buf-heaps.rst | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/Documentation/userspace-api/dma-buf-heaps.rst b/Documentation/userspace-api/dma-buf-heaps.rst > index 05445c83b79a..591732393e7d 100644 > --- a/Documentation/userspace-api/dma-buf-heaps.rst > +++ b/Documentation/userspace-api/dma-buf-heaps.rst > @@ -16,6 +16,14 @@ following heaps: > > - The ``system`` heap allocates virtually contiguous, cacheable, buffers. > > + - The ``system_cc_shared`` heap allocates virtually contiguous, cacheable, > + buffers using shared (decrypted) memory. It is only present on > + confidential computing (CoCo) VMs where memory encryption is active > + (e.g., AMD SEV, Intel TDX). The allocated pages have the encryption > + bit cleared, making them accessible for device DMA without TDISP > + support. On non-CoCo VMs configurations, this heap is "non-CoCo VM configurations" > + not registered. Doesn't seem like you need to wrap this line. with that: Reviewed-by: T.J.Mercier <tjmercier(a)google.com> > + > - The ``default_cma_region`` heap allocates physically contiguous, > cacheable, buffers. Only present if a CMA region is present. Such a > region is usually created either through the kernel commandline Each paragraph starting with '-' confused me for a second there. Those aren't part of the diff. :)

11 hours, 50 minutes

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Re: [PATCH 1/7] drm/shmem-helper: Add lockdep asserts to vmap/vunmap

by Christian König

On 4/2/26 18:13, Bence Csókás wrote: > [Sie erhalten nicht häufig E-Mails von bence.csokas(a)arm.com. Weitere Informationen, warum dies wichtig ist, finden Sie unter https://aka.ms/LearnAboutSenderIdentification ] > > Hi, > > I just came across this commit while researching something else. > Original patch had too few context lines, so I here's the diff with `-U10`. > > On 3/18/25 20:22, Daniel Almeida wrote: >> From: Asahi Lina <lina(a)asahilina.net> >> >> Since commit 21aa27ddc582 ("drm/shmem-helper: Switch to reservation >> lock"), the drm_gem_shmem_vmap and drm_gem_shmem_vunmap functions >> require that the caller holds the DMA reservation lock for the object. >> Add lockdep assertions to help validate this. > > There were already lockdep assertions... Good point, I completely missed that. > >> Signed-off-by: Asahi Lina <lina(a)asahilina.net> >> Signed-off-by: Daniel Almeida <daniel.almeida(a)collabora.com> >> Reviewed-by: Christian König <christian.koenig(a)amd.com> >> Signed-off-by: Lyude Paul <lyude(a)redhat.com> >> Link: https://lore.kernel.org/r/20250318-drm-gem-shmem-v1-1-64b96511a84f@collabor… >> --- >> drivers/gpu/drm/drm_gem_shmem_helper.c | 4 ++++ >> 1 file changed, 4 insertions(+) >> >> diff --git a/drivers/gpu/drm/drm_gem_shmem_helper.c b/drivers/gpu/drm/drm_gem_shmem_helper.c >> index aa43265f4f4f..0b41f0346bad 100644 >> --- a/drivers/gpu/drm/drm_gem_shmem_helper.c >> +++ b/drivers/gpu/drm/drm_gem_shmem_helper.c >> @@ -341,20 +341,22 @@ EXPORT_SYMBOL_GPL(drm_gem_shmem_unpin); >> * >> * Returns: >> * 0 on success or a negative error code on failure. >> */ >> int drm_gem_shmem_vmap_locked(struct drm_gem_shmem_object *shmem, >> struct iosys_map *map) >> { >> struct drm_gem_object *obj = &shmem->base; >> int ret = 0; >> >> + dma_resv_assert_held(obj->resv); >> + >> if (drm_gem_is_imported(obj)) { >> ret = dma_buf_vmap(obj->dma_buf, map); >> } else { >> pgprot_t prot = PAGE_KERNEL; >> >> dma_resv_assert_held(shmem->base.resv); > > ... right here, and > >> if (refcount_inc_not_zero(&shmem->vmap_use_count)) { >> iosys_map_set_vaddr(map, shmem->vaddr); >> return 0; >> @@ -401,20 +403,22 @@ EXPORT_SYMBOL_GPL(drm_gem_shmem_vmap_locked); >> * drops to zero. >> * >> * This function hides the differences between dma-buf imported and natively >> * allocated objects. >> */ >> void drm_gem_shmem_vunmap_locked(struct drm_gem_shmem_object *shmem, >> struct iosys_map *map) >> { >> struct drm_gem_object *obj = &shmem->base; >> >> + dma_resv_assert_held(obj->resv); >> + >> if (drm_gem_is_imported(obj)) { >> dma_buf_vunmap(obj->dma_buf, map); >> } else { >> dma_resv_assert_held(shmem->base.resv); > > ...here. > >> if (refcount_dec_and_test(&shmem->vmap_use_count)) { >> vunmap(shmem->vaddr); >> shmem->vaddr = NULL; >> >> drm_gem_shmem_unpin_locked(shmem); > > Or were those insufficient for some reason? If so, should we keep both > of them, or should the older ones have been removed? The dma_buf_vmap()/dma_buf_vunmap() functions require the caller to be holding the reservation lock as well. So it kind of makes sense to move the assertions to the beginning of the functions. Regards, Christian. > > Bence

4 days, 15 hours

[PATCH v4 0/8] dma-buf: heaps: Turn heaps into modules

by Maxime Ripard

Hi, The recent introduction of heaps in the optee driver [1] made possible the creation of heaps as modules. It's generally a good idea if possible, including for the already existing system and CMA heaps. The system one is pretty trivial, the CMA one is a bit more involved, especially since we have a call from kernel/dma/contiguous.c to the CMA heap code. This was solved by turning the logic around and making the CMA heap call into the contiguous DMA code. Let me know what you think, Maxime 1: https://lore.kernel.org/dri-devel/20250911135007.1275833-4-jens.wiklander@l… Signed-off-by: Maxime Ripard <mripard(a)kernel.org> --- Changes in v4: - Fix compilation failure - Rework to take into account OF_RESERVED_MEM - Fix regression making the default CMA area disappear if not created through the DT - Added some documentation and comments - Link to v3: https://lore.kernel.org/r/20260303-dma-buf-heaps-as-modules-v3-0-24344812c7… Changes in v3: - Squashed cma_get_name and cma_alloc/release patches - Fixed typo in Export dev_get_cma_area commit title - Fixed compilation failure with DMA_CMA but not OF_RESERVED_MEM - Link to v2: https://lore.kernel.org/r/20260227-dma-buf-heaps-as-modules-v2-0-454aee7e06… Changes in v2: - Collect tags - Don't export dma_contiguous_default_area anymore, but export dev_get_cma_area instead - Mentioned that heap modules can't be removed - Link to v1: https://lore.kernel.org/r/20260225-dma-buf-heaps-as-modules-v1-0-2109225a09… --- Maxime Ripard (8): dma: contiguous: Turn heap registration logic around dma: contiguous: Make dev_get_cma_area() a proper function dma: contiguous: Make dma_contiguous_default_area static dma: contiguous: Export dev_get_cma_area() mm: cma: Export cma_alloc(), cma_release() and cma_get_name() dma-buf: heaps: Export mem_accounting parameter dma-buf: heaps: cma: Turn the heap into a module dma-buf: heaps: system: Turn the heap into a module drivers/dma-buf/dma-heap.c | 1 + drivers/dma-buf/heaps/Kconfig | 4 +-- drivers/dma-buf/heaps/cma_heap.c | 22 +++---------- drivers/dma-buf/heaps/system_heap.c | 5 +++ include/linux/dma-buf/heaps/cma.h | 16 --------- include/linux/dma-map-ops.h | 14 ++++---- kernel/dma/contiguous.c | 66 +++++++++++++++++++++++++++++++++---- mm/cma.c | 3 ++ 8 files changed, 82 insertions(+), 49 deletions(-) --- base-commit: c081b71f11732ad2c443f170ab19c3ebe8a1a422 change-id: 20260225-dma-buf-heaps-as-modules-1034b3ec9f2a Best regards, -- Maxime Ripard <mripard(a)kernel.org>

4 days, 18 hours

Re: [PATCH v5 0/2] dma-buf: heaps: system: add an option to allocate explicitly shared/decrypted memory

by Brian Starkey

Hi, I know I'm late to the party here... Like John, I'm also not very close to this stuff any more, but I agree with the other discussions: makes sense for this to be a separate heap, and cc_shared makes sense too. I'm not clear why the heap depends on !CONFIG_HIGHMEM, but I also don't know anything about SEV/TDX. -Brian On Wed, Mar 25, 2026 at 08:23:50PM +0000, Jiri Pirko wrote: > From: Jiri Pirko <jiri(a)nvidia.com> > > Confidential computing (CoCo) VMs/guests, such as AMD SEV and Intel TDX, > run with private/encrypted memory which creates a challenge > for devices that do not support DMA to it (no TDISP support). > > For kernel-only DMA operations, swiotlb bounce buffering provides a > transparent solution by copying data through shared memory. > However, the only way to get this memory into userspace is via the DMA > API's dma_alloc_pages()/dma_mmap_pages() type interfaces which limits > the use of the memory to a single DMA device, and is incompatible with > pin_user_pages(). > > These limitations are particularly problematic for the RDMA subsystem > which makes heavy use of pin_user_pages() and expects flexible memory > usage between many different DMA devices. > > This patch series enables userspace to explicitly request shared > (decrypted) memory allocations from new dma-buf system_cc_shared heap. > Userspace can mmap this memory and pass the dma-buf fd to other > existing importers such as RDMA or DRM devices to access the > memory. The DMA API is improved to allow the dma heap exporter to DMA > map the shared memory to each importing device. > > Based on dma-mapping-for-next e7442a68cd1ee797b585f045d348781e9c0dde0d > > Jiri Pirko (2): > dma-mapping: introduce DMA_ATTR_CC_SHARED for shared memory > dma-buf: heaps: system: add system_cc_shared heap for explicitly > shared memory > > drivers/dma-buf/heaps/system_heap.c | 103 ++++++++++++++++++++++++++-- > include/linux/dma-mapping.h | 10 +++ > include/trace/events/dma.h | 3 +- > kernel/dma/direct.h | 14 +++- > kernel/dma/mapping.c | 13 +++- > 5 files changed, 132 insertions(+), 11 deletions(-) > > -- > 2.51.1 >

4 days, 20 hours

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