Re: [PATCH 0/5] vfio: Improve DMA mapping performance for huge pfnmaps

[Peter Xu]

On Wed, Feb 05, 2025 at 04:17:16PM -0700, Alex Williamson wrote:
> 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 (5):
>   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()
>   mm: Provide page mask in struct follow_pfnmap_args
>   vfio/type1: Use mapping page mask for pfnmaps

FWIW:

Reviewed-by: Peter Xu <peterx@xxxxxxxxxx>

Thanks,

-- 
Peter Xu