On Mon, Dec 29, 2025 at 10:38:26AM +1300, Barry Song wrote: > On Mon, Dec 29, 2025 at 3:49 AM Leon Romanovsky <[email protected]> wrote: > > > > On Sun, Dec 28, 2025 at 10:45:13AM +1300, Barry Song wrote: > > > On Sun, Dec 28, 2025 at 9:07 AM Leon Romanovsky <[email protected]> wrote: > > > > > > > > On Sat, Dec 27, 2025 at 11:52:44AM +1300, Barry Song wrote: > > > > > From: Barry Song <[email protected]> > > > > > > > > > > Currently, arch_sync_dma_for_cpu and arch_sync_dma_for_device > > > > > always wait for the completion of each DMA buffer. That is, > > > > > issuing the DMA sync and waiting for completion is done in a > > > > > single API call. > > > > > > > > > > For scatter-gather lists with multiple entries, this means > > > > > issuing and waiting is repeated for each entry, which can hurt > > > > > performance. Architectures like ARM64 may be able to issue all > > > > > DMA sync operations for all entries first and then wait for > > > > > completion together. > > > > > > > > > > To address this, arch_sync_dma_for_* now issues DMA operations in > > > > > batch, followed by a flush. On ARM64, the flush is implemented > > > > > using a dsb instruction within arch_sync_dma_flush(). > > > > > > > > > > For now, add arch_sync_dma_flush() after each > > > > > arch_sync_dma_for_*() call. arch_sync_dma_flush() is defined as a > > > > > no-op on all architectures except arm64, so this patch does not > > > > > change existing behavior. Subsequent patches will introduce true > > > > > batching for SG DMA buffers. > > > > > > > > > > Cc: Leon Romanovsky <[email protected]> > > > > > Cc: Catalin Marinas <[email protected]> > > > > > Cc: Will Deacon <[email protected]> > > > > > Cc: Marek Szyprowski <[email protected]> > > > > > Cc: Robin Murphy <[email protected]> > > > > > Cc: Ada Couprie Diaz <[email protected]> > > > > > Cc: Ard Biesheuvel <[email protected]> > > > > > Cc: Marc Zyngier <[email protected]> > > > > > Cc: Anshuman Khandual <[email protected]> > > > > > Cc: Ryan Roberts <[email protected]> > > > > > Cc: Suren Baghdasaryan <[email protected]> > > > > > Cc: Joerg Roedel <[email protected]> > > > > > Cc: Juergen Gross <[email protected]> > > > > > Cc: Stefano Stabellini <[email protected]> > > > > > Cc: Oleksandr Tyshchenko <[email protected]> > > > > > Cc: Tangquan Zheng <[email protected]> > > > > > Signed-off-by: Barry Song <[email protected]> > > > > > --- > > > > > arch/arm64/include/asm/cache.h | 6 ++++++ > > > > > arch/arm64/mm/dma-mapping.c | 4 ++-- > > > > > drivers/iommu/dma-iommu.c | 37 > > > > > +++++++++++++++++++++++++--------- > > > > > drivers/xen/swiotlb-xen.c | 24 ++++++++++++++-------- > > > > > include/linux/dma-map-ops.h | 6 ++++++ > > > > > kernel/dma/direct.c | 8 ++++++-- > > > > > kernel/dma/direct.h | 9 +++++++-- > > > > > kernel/dma/swiotlb.c | 4 +++- > > > > > 8 files changed, 73 insertions(+), 25 deletions(-) > > > > > > > > <...> > > > > > > > > > +#ifndef arch_sync_dma_flush > > > > > +static inline void arch_sync_dma_flush(void) > > > > > +{ > > > > > +} > > > > > +#endif > > > > > > > > Over the weekend I realized a useful advantage of the ARCH_HAVE_* config > > > > options: they make it straightforward to inspect the entire DMA path > > > > simply > > > > by looking at the .config. > > > > > > I am not quite sure how much this benefits users, as the same > > > information could also be obtained by grepping for > > > #define arch_sync_dma_flush in the source code. > > > > It differs slightly. Users no longer need to grep around or guess whether > > this > > platform used the arch_sync_dma_flush path. A simple grep for ARCH_HAVE_ in > > /proc/config.gz provides the answer. > > In any case, it is only two or three lines of code, so I am fine with > either approach. Perhaps Marek, Robin, and others have a point here? > > > > > > > > > > > > > > Thanks, > > > > Reviewed-by: Leon Romanovsky <[email protected]> > > > > > > Thanks very much, Leon, for reviewing this over the weekend. One thing > > > you might have missed is that I place arch_sync_dma_flush() after all > > > arch_sync_dma_for_*() calls, for both single and sg cases. I also > > > used a Python script to scan the code and verify that every > > > arch_sync_dma_for_*() is followed by arch_sync_dma_flush(), to ensure > > > that no call is left out. > > > > > > In the subsequent patches, for sg cases, the per-entry flush is > > > replaced by a single flush of the entire sg. Each sg case has > > > different characteristics: some are straightforward, while others > > > can be tricky and involve additional contexts. > > > > I didn't overlook it, and I understand your rationale. However, this is > > not how kernel patches should be structured. You should not introduce > > code in patch X and then move it elsewhere in patch X + Y. > > I am not quite convinced by this concern. This patch only > separates DMA sync issuing from completion waiting, and it > reflects that the development is done step by step. > > > > > Place the code in the correct location from the start. Your patches are > > small enough to review as is. > > My point is that this patch places the code in the correct locations > from the start. It splits arch_sync_dma_for_*() into > arch_sync_dma_for_*() plus arch_sync_dma_flush() everywhere, without > introducing any functional changes from the outset. > The subsequent patches clearly show which parts are truly batched. > > In the meantime, I do not have a strong preference here. If you think > it is better to move some of the straightforward batching code here, > I can follow that approach. Perhaps I could move patch 5, patch 8, > and the iommu_dma_iova_unlink_range_slow change from patch 7 here, > while keeping > > [PATCH 6] dma-mapping: Support batch mode for > dma_direct_{map,unmap}_sg > > and the IOVA link part from patch 7 as separate patches, since that > part is not straightforward. The IOVA link changes affect both > __dma_iova_link() and dma_iova_sync(), which are two separate > functions and require a deeper understanding of the contexts to > determine correctness. That part also lacks testing.
Don't worry about testing. NVME, RDMA and GPU are using this path and someone will test it. > > Would that be okay with you? I don't know, need to see the code. Thanks > > Thanks > Barry
