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

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Message-ID: <aV8ZRoDjKzjZaw5r@devgpu015.cco6.facebook.com>
Date: Wed, 7 Jan 2026 18:41:10 -0800
From: Alex Mastro <amastro@...com>
To: Jason Gunthorpe <jgg@...pe.ca>
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 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.

> 
> > Assuming not, should we add negative tests here to make sure iommufd
> > does not allow mapping BARs?
> 
> Yes

Will add.

>  
> > And then we can add dma-buf tests in a future commit.
> 
> Yes
> 
> Jason