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

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

Thanks Alex, this all looks great to me and completely eliminates the
boot time slowdown I was seeing in my tests on our DGX H100 and A100.
I also double-checked the memory mappings reported in /proc/iomem, and
everything looks consistent with how it was prior to this series on
both devices.

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


On Wed, Feb 5, 2025 at 5:18 PM Alex Williamson
<alex.williamson@redhat.com> 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@redhat.com/
>
>
> 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
>
>  drivers/vfio/vfio_iommu_type1.c | 107 ++++++++++++++++++++------------
>  include/linux/mm.h              |   2 +
>  mm/memory.c                     |   1 +
>  3 files changed, 72 insertions(+), 38 deletions(-)
>
> --
> 2.47.1
>


--
Mitchell Augustin
Software Engineer - Ubuntu Partner Engineering