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

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Message-ID: <aWBPNHOsaP1sNvze@google.com>
Date: Fri, 9 Jan 2026 00:43:32 +0000
From: David Matlack <dmatlack@...gle.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Alex Williamson <alex@...zbot.org>, Alex Mastro <amastro@...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 2026-01-08 08:36 PM, Jason Gunthorpe wrote:
> On Thu, Jan 08, 2026 at 09:29:29PM +0000, David Matlack wrote:
> > On 2026-01-08 02:33 PM, Jason Gunthorpe wrote:
> > > On Thu, Jan 08, 2026 at 10:24:19AM -0800, David Matlack wrote:
> > > > > > 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,
> > > > 
> > > > Lack of huge mappings in the IOMMU when using VFIO_TYPE1_IOMMU is
> > > > another gap I'm aware of.
> > > > vfio_dma_mapping_test.vfio_type1_iommu_anonymous_hugetlb_1gb.dma_map_unmap
> > > > fails when IOMMUFD_VFIO_CONTAINER is enabled.
> > > 
> > > What is this? I'm not aware of it..
> > 
> > It's one of the test cases within
> > tools/testing/selftests/vfio/vfio_dma_mapping_test.c.
> > 
> > Here's the output when running with CONFIG_IOMMUFD_VFIO_CONTAINER=y:
> > 
> >   #  RUN           vfio_dma_mapping_test.vfio_type1_iommu_anonymous_hugetlb_1gb.dma_map_unmap ...
> >   Mapped HVA 0x7f0480000000 (size 0x40000000) at IOVA 0x0
> >   Searching for IOVA 0x0 in /sys/kernel/debug/iommu/intel/0000:6a:01.0/domain_translation_struct
> >   Found IOMMU mappings for IOVA 0x0:
> >   PGD: 0x0000000203475027
> >   P4D: 0x0000000203476027
> >   PUD: 0x0000000203477027
> >   PMD: 0x00000001e7562027
> >   PTE: 0x00000041c0000067
> >   # tools/testing/selftests/vfio/vfio_dma_mapping_test.c:188:dma_map_unmap:Expected 0 (0) == mapping.pte (282394099815)
> >   # dma_map_unmap: Test terminated by assertion
> >   #          FAIL  vfio_dma_mapping_test.vfio_type1_iommu_anonymous_hugetlb_1gb.dma_map_unmap
> 
> I can't think of any reason this would fail, I think your tests have
> found a real bug?? Can you check into it, what kernel call fails and
> where does the kernel code come from?

Oh I thought it was by design. This code in iommufd_vfio_set_iommu():

	/*
	 * The difference between TYPE1 and TYPE1v2 is the ability to unmap in
	 * the middle of mapped ranges. This is complicated by huge page support
	 * which creates single large IOPTEs that cannot be split by the iommu
	 * driver. TYPE1 is very old at this point and likely nothing uses it,
	 * however it is simple enough to emulate by simply disabling the
	 * problematic large IOPTEs. Then we can safely unmap within any range.
	 */
	if (type == VFIO_TYPE1_IOMMU)
		rc = iopt_disable_large_pages(&ioas->iopt);

git-blame says some guy named Jason Gunthorpe wrote it :P

> 
> I don't think I can run these tests with the HW I have??

FWIW all you need any PCI device that can be bound to vfio-pci and
mapped by VT-d. This test does not rely on any of the VFIO selftests
drivers to trigger DMA.