Re: exynos-mixer 14450000.mixer: [drm:exynos_drm_register_dma] *ERROR* Device 14450000.mixer lacks support for IOMMU

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

Unfortunately there is no easy solution.

Do you know the version of the SMMU available on the platform? If it is
a SMMUv3 you can try to use the nested SMMU patch series to enable a
virtual SMMU in Dom0: https://marc.info/?l=xen-devel&m=166991020831005
That way, Xen can use the SMMU to protect VMs, and Dom0 can also use the
SMMU for its own purposes at the same time.

Alternatively, you can dig into the details of the exynos-drm driver to
see what exactly is the dependency on the IOMMU framework in Linux and
remove the dependency. Unfortunately none of us in this thread are
expert on exynos-drm so it would be difficult to advise on how to do
that. For example, I don't know how you could debug the x11 problem you
described because I don't typically work with x11 or with the exynos. If
there is an open source mailing list for exynos-drm development they
might be able to advise on how to remove the IOMMU dependency there.

The final option, which is a gross hack, would be to let Dom0 use the
IOMMU for its own gain. Xen doesn't use the IOMMU. If you do that you
lose freedom from interference between the VMs and you cannot run driver
domains or directly assign devices to DomUs. But if you are running a
research project you might be OK with that. To get it to work, you need
to hack Xen so that it remaps the IOMMU to Dom0 to let Dom0 program it
directly. The attached patch (untested) would be a place to start. You
also need to pass iommu=false to the Xen command line to prevent Xen
from using the iommu itself.

Cheers,

Stefano


On Wed, 1 Nov 2023, Mario Marietto wrote:
> I'm aware of the presence of that post. I'm working on the same
> project with the guy who explained the problem. Unfortunately,the
> solution proposed does not work well. Xen is working,but the screen is
> still black.
> 
> On Wed, Nov 1, 2023 at 12:04 AM Stefano Stabellini
> <sstabellini@xxxxxxxxxx> wrote:
> >
> > Hi Mario,
> >
> > I am adding xen-devel and a couple of other Xen maintainers that might
> > know how to help make progress on this issues.
> >
> > Replies inline below.
> >
> >
> > On Tue, 31 Oct 2023, Mario Marietto wrote:
> > > Hello,
> > >
> > > We are a team of linux enthusiasts who are trying to boot Xen on a
> > > Samsung XE303C12 Chromebook aka "snow"
> > > following the suggestions in the slide show presentation here:
> > >
> > > https://www.slideshare.net/xen_com_mgr/xpds16-porting-xen-on-arm-to-a-new-soc-julien-grall-arm
> > >
> > > This device uses an exynos5250 SOC dual core 1.7 GHz with 2 MB RAM, it is
> > > a Samsung armv7 chip with virtualization extensions.
> > >
> > > In particular, we have it working fairly well both on the bare metal with
> > > a recent 6.1.59 Linux LTS kernel and also with a recent 5.4.257 LTS
> > > kernel with KVM, the older LTS kernel version is used to test KVM because
> > > support for KVM on arm v7 was removed from Linux around kernel version
> > > 5.7. So we know we have the hypervisor mode enabled because we were able
> > > to use it with KVM.
> > >
> > > For Xen, we are using the latest Debian build of Xen 4.17 for the Debian
> > > armhf architecture:
> > >
> > > (XEN) Xen version 4.17.2-pre (Debian 4.17.1+2-gb773c48e36-1)
> > > (pkg-xen-devel@xxxxxxxxxxxxxxx
> > > xxxxxxxx) (arm-linux-gnueabihf-gcc (Debian
> > > 12.2.0-14) 12.2.0) debug=n Thu May 18 19:26:30 UTC 2023
> > >
> > > The Linux kernel is a custom build that adds the Xen config kernel
> > > options (CONFIG_XEN_DOM0, etc) on top of a kernel that works well on the
> > > same Chromebook model on the bare metal. I can provide the config options
> > > of the kernel that was used if that is helpful.
> > >
> > > Our method of booting is to have u-boot boot the Xen hypervisor and load
> > > the device tree after adding the dom0 to the otherwise unaltered device
> > > tree from the Linux kernel using u-boot fdt commands to add a /chosen
> > > node, as described on the Xen wiki and in the pages linked from there. We
> > > have also tried adding and loading an initrd.img using the device tree
> > > /chosen node but that made no difference in our tests.
> > >
> > > We actually have the Linux LTS kernel version 6.1.59 working as dom0 with
> > > Xen using the same version of u-boot that we used for KVM, but with a big
> > > problem.
> > >
> > > The problem we see is that when booting the 6.1.59 kernel version as dom0
> > > with Xen, the screen is totally dark and the only way to access the
> > > system is remotely through ssh. Logs indicate most everything else is
> > > working, such as the wifi card so we can access it remotely via ssh and a
> > > USB optical mouse lights up when connected so USB is also working.
> > > Obviously, the disk is also working. The Chromebook is configured to boot
> > > from the device's SD card slot by turning on Chrome OS developer mode
> > > options to enable booting from the SD card slot.
> > >
> > > The mystery is that when booting the exact same 6.1.59 kernel on the bare
> > > metal instead of booting it as dom0 on Xen, it boots up with full access
> > > to the screen and we can interact with the system using the X.org windows
> > > system. But booting as dom0 with Xen, the screen is totally dark and the
> > > only access we have to the system is through the network via ssh. Also,
> > > when booting the 5.4.257 kernel with KVM in hypervisor mode, the screen
> > > works and we can interact with the system through the X.org windows
> > > system.
> > >
> > > Exploring the log file,we have seen the errors below :
> > >
> > >
> > > With Xen (or in bare metal):
> > >
> > > devuan-bunsen kernel: [drm] Exynos DRM: using 14400000.fimd device for
> > > DMA mapping operations
> > > devuan-bunsen kernel: exynos-drm exynos-drm: bound 14400000.fimd (ops
> > > 0xc0d96354)
> > > devuan-bunsen kernel: exynos-drm exynos-drm: bound 14450000.mixer (ops
> > > 0xc0d97554)
> > > devuan-bunsen kernel: exynos-drm exynos-drm: bound
> > > 145b0000.dp-controller (ops 0xc0d97278)
> > > devuan-bunsen kernel: exynos-drm exynos-drm: bound 14530000.hdmi (ops
> > > 0xc0d97bd0)
> > > ...
> > > devuan-bunsen kernel: Console: switching to colour frame buffer device 
> > > 170x48
> > > devuan-bunsen kernel: exynos-drm exynos-drm: [drm] fb0: exynosdrmfb
> > > frame buffer device
> > > devuan-bunsen kernel: [drm] Initialized exynos 1.1.0 20180330 for
> > > exynos-drm on minor 0
> > >
> > > In this case,the kernel is able to use the exynos-drm kernel to start
> > > the fb0 device. But with Xen we get this error with exynos-drm:
> > >
> > > devuan-bunsen kernel: [drm] Exynos DRM: using 14400000.fimd device for
> > > DMA mapping operations
> > > devuan-bunsen kernel: exynos-drm exynos-drm: bound 14400000.fimd (ops
> > > 0xc0d96354)
> > > devuan-bunsen kernel: exynos-mixer 14450000.mixer:
> > > [drm:exynos_drm_register_dma] *ERROR* Device 14450000.mixer lacks
> > > support for IOMMU
> > > devuan-bunsen kernel: exynos-drm exynos-drm: failed to bind
> > > 14450000.mixer (ops 0xc0d97554): -22
> > > devuan-bunsen kernel: exynos-drm exynos-drm: adev bind failed: -22
> > > devuan-bunsen kernel: exynos-dp: probe of 145b0000.dp-controller
> > > failed with error -22
> > >
> > >
> > > Any ideas why booting the same Linux kernel that results in a working
> > > X.org display on the bare metal instead as dom0 on Xen would cause the
> > > display to remain dark, but most other basic functions would work, such
> > > as network, disk, and USB ? thanks.
> >
> > I think the issue is that Linux 6.1.59 is using the IOMMU to get the GPU
> > to work. When running on top of Xen, the IOMMU is not available to
> > Linux. That's because the IOMMU is used by Xen to enforce protection
> > between VMs.
> >
> > It might help to rebuild Linux without IOMMU support (remove the driver
> > from the kconfig) so that Linux cannot attempt to use the IOMMU.
> >
> > Also see
> > https://marc.info/?l=xen-users&m=169844593525492

Attachment: dom0-iommu.patch
Description: Text Data