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Message-ID: <CAHTA-uZciMceCK3OF6krjoPx1kaoeQRuYhkpfZ_tZJQ4dmhzJQ@mail.gmail.com>
Date: Tue, 18 Feb 2025 20:37:28 -0600
From: Mitchell Augustin <mitchell.augustin@...onical.com>
To: Alex Williamson <alex.williamson@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, peterx@...hat.com,
clg@...hat.com, jgg@...dia.com, akpm@...ux-foundation.org, linux-mm@...ck.org,
david@...hat.com, willy@...radead.org
Subject: Re: [PATCH v2 0/6] vfio: Improve DMA mapping performance for huge pfnmaps
No change in behavior observed from v1 on my config (DGX H100). Thanks!
Reviewed-by: "Mitchell Augustin" <mitchell.augustin@...onical.com>
Tested-by: "Mitchell Augustin" <mitchell.augustin@...onical.com>
On Tue, Feb 18, 2025 at 4:22 PM Alex Williamson
<alex.williamson@...hat.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@...onical.com
Location:United States of America
canonical.com
ubuntu.com
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