Re: [PATCH v2 0/6] vfio: Improve DMA mapping performance for huge pfnmaps

[Alex Williamson]

On Tue, 18 Feb 2025 15:22:00 -0700
Alex Williamson <alex.williamson@xxxxxxxxxx> wrote:

> v2:
>  - Rewrapped comment block in 3/6
>  - Added 4/6 to use consistent types (Jason)
>  - Renamed s/pgmask/addr_mask/ (David)
>  - Updated 6/6 with proposed epfn algorithm (Jason)
>  - Applied and retained sign-offs for all but 6/6 where the epfn
>    calculation changed
> 
> v1: https://lore.kernel.org/all/20250205231728.2527186-1-alex.williamson@xxxxxxxxxx/
> 
> As GPU BAR sizes increase, the overhead of DMA mapping pfnmap ranges has
> become a significant overhead for VMs making use of device assignment.
> Not only does each mapping require upwards of a few seconds, but BARs
> are mapped in and out of the VM address space multiple times during
> guest boot.  Also factor in that multi-GPU configurations are
> increasingly commonplace and BAR sizes are continuing to increase.
> Configurations today can already be delayed minutes during guest boot.
> 
> We've taken steps to make Linux a better guest by batching PCI BAR
> sizing operations[1], but it only provides and incremental improvement.
> 
> This series attempts to fully address the issue by leveraging the huge
> pfnmap support added in v6.12.  When we insert pfnmaps using pud and pmd
> mappings, we can later take advantage of the knowledge of the mapping
> level page mask to iterate on the relevant mapping stride.  In the
> commonly achieved optimal case, this results in a reduction of pfn
> lookups by a factor of 256k.  For a local test system, an overhead of
> ~1s for DMA mapping a 32GB PCI BAR is reduced to sub-millisecond (8M
> page sized operations reduced to 32 pud sized operations).
> 
> Please review, test, and provide feedback.  I hope that mm folks can
> ack the trivial follow_pfnmap_args update to provide the mapping level
> page mask.  Naming is hard, so any preference other than pgmask is
> welcome.  Thanks,
> 
> Alex
> 
> [1]https://lore.kernel.org/all/20250120182202.1878581-1-alex.williamson@xxxxxxxxxx/
> 
> 
> Alex Williamson (6):
>   vfio/type1: Catch zero from pin_user_pages_remote()
>   vfio/type1: Convert all vaddr_get_pfns() callers to use vfio_batch
>   vfio/type1: Use vfio_batch for vaddr_get_pfns()
>   vfio/type1: Use consistent types for page counts
>   mm: Provide address mask in struct follow_pfnmap_args
>   vfio/type1: Use mapping page mask for pfnmaps
> 
>  drivers/vfio/vfio_iommu_type1.c | 123 ++++++++++++++++++++------------
>  include/linux/mm.h              |   2 +
>  mm/memory.c                     |   1 +
>  3 files changed, 80 insertions(+), 46 deletions(-)

With David's blessing relative to mm, applied to vfio next branch for
v6.15.  Thanks all for the reviews and testing!

Alex