linux-kernel - Re: [PATCH v2 6/6] vfio/type1: Use mapping page mask for pfnmaps

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Message-ID: <20250225001752.GL520155@nvidia.com>
Date: Mon, 24 Feb 2025 20:17:52 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Alex Williamson <alex.williamson@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, peterx@...hat.com,
	mitchell.augustin@...onical.com, clg@...hat.com,
	willy@...radead.org
Subject: Re: [PATCH v2 6/6] vfio/type1: Use mapping page mask for pfnmaps

On Tue, Feb 18, 2025 at 03:22:06PM -0700, Alex Williamson wrote:
> vfio-pci supports huge_fault for PCI MMIO BARs and will insert pud and
> pmd mappings for well aligned mappings.  follow_pfnmap_start() walks the
> page table and therefore knows the page mask of the level where the
> address is found and returns this through follow_pfnmap_args.pgmask.
> Subsequent pfns from this address until the end of the mapping page are
> necessarily consecutive.  Use this information to retrieve a range of
> pfnmap pfns in a single pass.
> 
> With optimal mappings and alignment on systems with 1GB pud and 4KB
> page size, this reduces iterations for DMA mapping PCI BARs by a
> factor of 256K.  In real world testing, the overhead of iterating
> pfns for a VM DMA mapping a 32GB PCI BAR is reduced from ~1s to
> sub-millisecond overhead.
> 
> Signed-off-by: Alex Williamson <alex.williamson@...hat.com>
> ---
>  drivers/vfio/vfio_iommu_type1.c | 23 ++++++++++++++++-------
>  1 file changed, 16 insertions(+), 7 deletions(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason