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

Alex Williamson
2025-02-18T22:22:35Z

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@redhat.com/

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@redhat.com/

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(-)

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2.48.1