[PATCH 0/7] Improve swiotlb performance by using physical addresses

[Alexander Duyck]

While working on 10Gb/s routing performance I found a significant amount of
time was being spent in the swiotlb DMA handler.  Further digging found that a
significant amount of this was due to virtual to physical address translation
and calling the function that did it.  It accounted for nearly 60% of the
total swiotlb overhead.

This patch set works to resolve that by replacing the io_tlb_start virtual
address with io_tlb_addr which is a physical address.  In addition it changes
the io_tlb_overflow_buffer from a virtual to a physical address.  I followed
through with the cleanup to the point that the only functions that really
require the virtual address for the DMA buffer are the init, free, and
bounce functions.

In the case of devices that are using the bounce buffers these patches should
result in only a slight performance gain if any.  This is due to the locking
overhead required to map and unmap the buffers.

In the case of devices that are not making use of bounce buffers these patches
can significantly reduce their overhead.  In the case of an ixgbe routing test
for example, these changes result in 7 fewer calls to __phys_addr and
allow is_swiotlb_buffer to become inlined due to a reduction in the number of
instructions.  When running a routing throughput test using small packets I
saw roughly a 5% increase in packets rates after applying these patches.  This
appears to match up with the CPU overhead reduction I was tracking via perf.

Before:
Results 10.29Mps
# Overhead Symbol
# ........ ...........................................................................................................
#
     1.97%  [k] __phys_addr                                                                                   
            |              
            |--24.97%-- swiotlb_sync_single
            |          
            |--16.55%-- is_swiotlb_buffer
            |          
            |--11.25%-- unmap_single
            |          
             --2.71%-- swiotlb_dma_mapping_error
     1.66%  [k] swiotlb_sync_single                                                                                 
     1.45%  [k] is_swiotlb_buffer                                     
     0.53%  [k] unmap_single                                                                                     
     0.52%  [k] swiotlb_map_page                                      
     0.47%  [k] swiotlb_sync_single_for_device                                                                 
     0.43%  [k] swiotlb_sync_single_for_cpu                           
     0.42%  [k] swiotlb_dma_mapping_error                                                               
     0.34%  [k] swiotlb_unmap_page                

After:
Results 10.99Mps
# Overhead Symbol
# ........ ...........................................................................................................
#
     0.50%  [k] swiotlb_map_page                                                                                        
     0.50%  [k] swiotlb_sync_single                                                                                     
     0.36%  [k] swiotlb_sync_single_for_cpu                                                                             
     0.35%  [k] swiotlb_sync_single_for_device                                                                          
     0.25%  [k] swiotlb_unmap_page                                                                                      
     0.17%  [k] swiotlb_dma_mapping_error  

Finally, I updated the parameter names for several of the core function calls
as there was some ambiguity in naming.  Specifically virtual address pointers
were named dma_addr.  When I changed these pointers to physical I instead used
the name tlb_addr as this value represented a physical address in the
io_tlb_addr region and is less likely to be confused with a bus address.

---

Alexander Duyck (7):
      swiotlb:  Do not export swiotlb_bounce since there are no external consumers
      swiotlb: Use physical addresses instead of virtual in swiotlb_tbl_sync_single
      swiotlb: Use physical addresses for swiotlb_tbl_unmap_single
      swiotlb: Return physical addresses when calling swiotlb_tbl_map_single
      swiotlb: Make io_tlb_overflow_buffer a physical address
      swiotlb: Replace virtual io_tlb_start with physical io_tlb_addr
      swiotlb: Instead of tracking the end of the swiotlb region just calculate it


 drivers/xen/swiotlb-xen.c |   25 ++--
 include/linux/swiotlb.h   |   20 ++-
 lib/swiotlb.c             |  285 +++++++++++++++++++++++----------------------
 3 files changed, 170 insertions(+), 160 deletions(-)

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