[RFC v2 0/2] swiotlb performance optimizations

[Chao Gao]

Intent of this post:
 Seek reviews from Intel reviewers and anyone else in the list
 interested in IO performance in confidential VMs. Need some acked-by
 reviewed-by tags before I can add swiotlb maintainers to "to/cc" lists
 and ask for a review from them.

Changes from v1 to v2:
- rebase to the latest dma-mapping tree.
- drop the duplicate patch for mitigating lock contention
- re-collect perf data

swiotlb is now widely used by confidential VMs. This series optimizes
swiotlb to reduce cache misses and lock contention during bounce buffer
allocation/free and memory bouncing to improve IO workload performance in
confidential VMs.

Here are some FIO tests we did to demonstrate the improvement.

Test setup
----------

A normal VM with 8vCPU and 32G memory, swiotlb is enabled by swiotlb=force.
FIO block size is 4K and iodepth is 256. Note that a normal VM is used so
that others lack of necessary hardware to host confidential VMs can reproduce
results below.

Results
-------

1 FIO job	read/write	IOPS (k)
vanilla		read		216 
		write		251 
optimized	read		250 
		write		270 

1-job FIO sequential read/write perf increase by 19% and 8% respectively.

Chao Gao (2):
  swiotlb: use bitmap to track free slots
  swiotlb: Allocate memory in a cache-friendly way

 include/linux/swiotlb.h |   8 ++-
 kernel/dma/swiotlb.c    | 127 +++++++++++++++++-----------------------
 2 files changed, 60 insertions(+), 75 deletions(-)

-- 
2.25.1