linux-kernel - Re: [PATCH] vfio: selftests: Add vfio_dma_mapping_mmio

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Message-ID: <20260108141044.GC545276@ziepe.ca>
Date: Thu, 8 Jan 2026 10:10:44 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Alex Mastro <amastro@...com>
Cc: David Matlack <dmatlack@...gle.com>, Alex Williamson <alex@...zbot.org>,
	Shuah Khan <shuah@...nel.org>, Peter Xu <peterx@...hat.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	linux-kselftest@...r.kernel.org
Subject: Re: [PATCH] vfio: selftests: Add vfio_dma_mapping_mmio_test

On Wed, Jan 07, 2026 at 06:41:10PM -0800, Alex Mastro wrote:
> On Wed, Jan 07, 2026 at 08:54:06PM -0400, Jason Gunthorpe wrote:
> > On Wed, Jan 07, 2026 at 11:54:09PM +0000, David Matlack wrote:
> > > On 2026-01-07 02:13 PM, Alex Mastro wrote:
> > > > Test MMIO-backed DMA mappings by iommu_map()-ing mmap'ed BAR regions.
> > > 
> > > Thanks for adding this!
> > > 
> > > > Also update vfio_pci_bar_map() to align BAR mmaps for efficient huge
> > > > page mappings.
> > > > 
> > > > Only vfio_type1 variants are tested; iommufd variants can be added
> > > > once kernel support lands.
> > > 
> > > Are there plans to support mapping BARs via virtual address in iommufd?
> > > I thought the plan was only to support via dma-bufs. Maybe Jason can
> > > confirm.
> > 
> > Only dmabuf.
> 
> Ack. I got confused. I had thought iommufd's vfio container compatibility mode
> was going to support this, but realized that doesn't make sense given past
> discussions about the pitfalls of achieving these mappings the legacy way.

Oh, I was thinking about a compatability only flow only in the type 1
emulation that internally magically converts a VMA to a dmabuf, but I
haven't written anything.. It is a bit tricky and the type 1 emulation
has not been as popular as I expected??

Jason