RFC iommutests_: Testing software for everything IOMMU

[Jean-Philippe Brucker]

On Fri, Mar 28, 2025 at 10:11:13AM +0100, Joel Granados wrote:
> Custom qemu device: pci-ats-testdev
> -------------------------------------
> To support IOMMU testing under qemu, the pci-ats-testdev [10]
> (different from pci-testdev [11]) was used to emulate DMA transactions.
> It is a full fledged pci device capable of executing emulated DMA
> accesses. It was originally intended to test Linux kernel interactions
> with devices that had a working Address Translation Cache (ATC) but can
> become a platform capable of testing anything PCI/IOMMU related if
> needed.

Yes please!  Maybe "pcie-testdev" rather than "pci-ats-testdev"?  There
are other PCIe features that are poorly tested at the moment, for example
PASID and PRI. The programming model of devices that actually implement
those can get too complex so we need something simpler to precisely stress
the IOMMU driver infrastructure. Driver unit-tests alone aren't good
enough for exercising TLB invalidation (DMA after removing a mapping must
crash), tricky cleanup paths (eg. killing a process bound to a device
that's issuing page requests), runtime PM, MSIs etc. I'm guessing testing
newer/future features like TDISP would also benefit from a simple device.

Some time back I needed a device like that to reproduce some tricky races
but never got round to implementing extra PCIe features. Although this one
[1] is based on virtio any programming interface should work as long as it
can instruct the device to send precise DMA transactions, ideally many in
parallel.

Thanks,
Jean

[1] https://jpbrucker.net/git/linux/log/?h=virtio-dmatest/latest
    https://jpbrucker.net/git/qemu/log/?h=virtio-dmatest/latest