Re: [GSoC 2026] Userland PCI drivers

[Oliver Miyar Ugarte <olivermu%estudante.ufscar.br@localhost>]

Hi!

Michael <macallan1888%gmail.com@localhost> wrote:
>
> Hello,
>
> On Tue, 3 Mar 2026 18:48:36 -0300
> Oliver Miyar Ugarte <olivermu%estudante.ufscar.br@localhost> wrote:
>
> > I've been working on the Userland PCI Drivers project for GSoC 2026
> > (https://wiki.netbsd.org/projects/project/userland_pci/) and have a
> > draft implementation of the first milestone, achieved by mapping PCI
> > BARs from userspace via a new ioctl.
> > (https://github.com/NetBSD/src/pull/74)
> >
> > This adds PCI_IOC_MAP_BAR to /dev/pci/pci_usrreq.c, allowing userspace
> > to safely map device registers without using /dev/mem. I've tested it
> > with QEMU's edu device and it returns the correct BAR offset and size.
>
> You can already map PCI resources by their bus addresses via /dev/pci*,
> and access config space via ioctl(PCI_IOC_BDF_CFG{READ|WRITE}).
> That's what the Xserver uses.
> See
> https://cvsweb.netbsd.org/bsdweb.cgi/xsrc/external/mit/libpciaccess/dist/src/netbsd_pci.c?rev=1.23
> and https://cvsweb.netbsd.org/bsdweb.cgi/src/lib/libpci/
>
> What's missing is stuff like DMA and interrupts from userland.
>
> No idea why the project proposal mentions /dev/mem at all - it's not
> portable ( there's a lot of supported hardware where PCI bus addresses
> do not map 1:1 to physical addresses in CPU space, and others where you
> can only see actual RAM through /dev/mem, not PCI space ) and requires
> knowledge of the underlying hardware other than the device you're
> trying to talk to.
>
> So, why the additional ioctl? You can already access config space, find
> devices and their BARs, and mmap() them at offset == bus address
> without any kernel changes.
>
> have fun
> Michael


Thanks a lot for the feedback!

I can't believe I missed that existing infrastructure, I had tunnel
vision on doing what the project proposal mentioned and didn't check
sufficiently if it already existed.

I will focus my project on adding DMA and interrupts to userland since
that's what's needed.
Do you have any advice on that?

Well wishes!
Oliver Miyar Ugarte