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

[Mitchell Augustin <mitchell.augustin@canonical.com>]

No change in behavior observed from v1 on my config (DGX H100). Thanks!

Reviewed-by: "Mitchell Augustin" <mitchell.augustin@canonical.com>
Tested-by: "Mitchell Augustin" <mitchell.augustin@canonical.com>

On Tue, Feb 18, 2025 at 4:22 PM Alex Williamson
<alex.williamson@redhat.com> 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@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(-)
>
> --
> 2.48.1
>


-- 
Mitchell Augustin
Software Engineer - Ubuntu Partner Engineering
Email:mitchell.augustin@canonical.com
Location:United States of America


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