On 07/06/2022 23:43, Bart Van Assche wrote:
On 6/6/22 02:30, John Garry wrote:
As reported in [0], DMA mappings whose size exceeds the IOMMU IOVA
caching
limit may see a big performance hit.
This series introduces a new DMA mapping API, dma_opt_mapping_size(), so
that drivers may know this limit when performance is a factor in the
mapping.
Robin didn't like using dma_max_mapping_size() for this [1].
The SCSI core code is modified to use this limit.
I also added a patch for libata-scsi as it does not currently honour the
shost max_sectors limit.
Note: Christoph has previously kindly offered to take this series via the
dma-mapping tree, so I think that we just need an ack from the
IOMMU guys now.
[0]
https://lore.kernel.org/linux-iommu/20210129092120.1482-1-thunder.leizhen@xxxxxxxxxx/
[1]
https://lore.kernel.org/linux-iommu/f5b78c9c-312e-70ab-ecbb-f14623a4b6e3@xxxxxxx/
Regarding [0], that patch reverts commit 4e89dce72521 ("iommu/iova:
Retry from last rb tree node if iova search fails"). Reading the
description of that patch, it seems to me that the iova allocator can be
improved. Shouldn't the iova allocator be improved such that we don't
need this patch series? There are algorithms that handle fragmentation
much better than the current iova allocator algorithm, e.g. the
https://en.wikipedia.org/wiki/Buddy_memory_allocation algorithm.
Regardless of whether the IOVA allocator can be improved - which it
probably can be - this series is still useful. That is due to the IOVA
rcache - that is a cache of pre-allocated IOVAs which can be quickly
used in the DMA mapping. The rache contains IOVAs up to certain fixed
size. In this series we limit the DMA mapping length to the rcache size
upper limit to always bypass the allocator (when we have a cached IOVA
available) - see alloc_iova_fast().
Even if the IOVA allocator were greatly optimised for speed, there would
still be an overhead in the alloc and free for those larger IOVAs which
would outweigh the advantage of having larger DMA mappings. But is there
even an advantage in very large streaming DMA mappings? Maybe for iotlb
efficiency. But some say it's better to have the DMA engine start
processing the data ASAP and not wait for larger lists to be built.
Thanks,
John
