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@xxxxxxxxxxxxx> Reviewed-by: "Mitchell Augustin" <mitchell.augustin@xxxxxxxxxxxxx> Tested-by: "Mitchell Augustin" <mitchell.augustin@xxxxxxxxxxxxx> On Wed, Feb 5, 2025 at 5:18 PM Alex Williamson <alex.williamson@xxxxxxxxxx> 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 > > 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
