Messages in this thread | ![/](/images/icornerl.gif) | | From | Michael Kelley <> | Subject | RE: [PATCH v5 6/6] swiotlb: Remove pointless stride adjustment for allocations >= PAGE_SIZE | Date | Thu, 29 Feb 2024 15:44:11 +0000 |
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From: Christoph Hellwig <hch@lst.de> Sent: Thursday, February 29, 2024 5:34 AM > > On Thu, Feb 29, 2024 at 07:36:05AM +0000, Michael Kelley wrote: > > If there *is* a requirement for page alignment of page-size-or-greater > > requests, even when alloc_align_mask and min_align_mask are zero, > > we need to think about how to do that correctly, as that requirement > > is no longer met after Patch 1 of this series. > > It has been historical behavior that all dma allocations are page > aligned (including in fact smaller than page sized ones that get > rounded up to a page size). The documentation actually (incorretly) > states an even stronger guarantee: > > "The CPU virtual address and the DMA address are both > guaranteed to be aligned to the smallest PAGE_SIZE order which > is greater than or equal to the requested size. This invariant > exists (for example) to guarantee that if you allocate a chunk > which is smaller than or equal to 64 kilobytes, the extent of the > buffer you receive will not cross a 64K boundary."
Any thoughts on how that historical behavior should apply if the DMA min_align_mask is non-zero, or the alloc_align_mask parameter to swiotbl_tbl_map_single() is non-zero? As currently used, alloc_align_mask is page aligned if the IOMMU granule is >= PAGE_SIZE. But a non-zero min_align_mask could mandate returning a buffer that is not page aligned. Perhaps do the historical behavior only if alloc_align_mask and min_align_mask are both zero?
Michael
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