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

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Message-ID: <20260108084514.1d5e3ee3@shazbot.org>
Date: Thu, 8 Jan 2026 08:45:14 -0700
From: Alex Williamson <alex@...zbot.org>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Alex Mastro <amastro@...com>, David Matlack <dmatlack@...gle.com>, 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 Thu, 8 Jan 2026 10:10:44 -0400
Jason Gunthorpe <jgg@...pe.ca> wrote:

> 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??

In part because of this gap, I'd guess.  Thanks,

Alex