[vfio-users] DMA Error -14 with I2C controller passthrough

[Davide Miola]

Hi,

I'm trying to passthrough one of the two I2C controllers my laptop has to
get the touchpad working in a VM (Disclaimer: I know it's not as simple as
passing through the controller, but I've got to start somewhere).

I saw this
<https://www.mail-archive.com/vfio-users@redhat.com/msg03702.html> thread,
so I think it should be possible after all, but so far I'm facing a lot of
trouble:
basically I can't get any of the two I2C controllers of my laptop to
passthrough, because, after binding them to vfio-pci and starting a VM (I'm
using a Fedora Workstation iso for that), I get this error:

qemu-system-x86_64: VFIO_MAP_DMA: -14
qemu-system-x86_64: vfio_dma_map(0x7f1cccdf0fc0, 0xfebf1000, 0x1000,
0x7f1ccf493000) = -14 (Wrong address)

I get that once when SeaBIOS loads and then another six times in a burst
when the guest os loads (the amount of times I get that error seems to
change when using libvirt).
Dmesg says nothing both on the host and the guest.
This happens both using libvirt and without, but I've also tried:
 - 5.4 instead of 5.3 kernel;
 - Manjaro and Fedora host;
 - booting in legacy instead of UEFI;
 - binding to vfio-pci via kernel parameters instead of doing it after boot;
 - using different QEMU machines;
 - using OVMF or SeaBIOS;
 - using KVM or not.

None of this made any difference.

I should clarify that that error does not prevent the VM from booting, in
fact it seems to boot just fine, and lspci on the guest even reports the
device as being present (same name and vendor/device ids as on the host),
but it won't bind to intel-lpss, that's the real problem.

I should also mention that I have no problems passing through other devices
on this laptop, in fact I've successfully passed through the IGPU (legacy
passthrough), the USB controller, and chipset audio, all without a problem.

It goes without saying that I'd be very interested in a possible future
upstream implementation of something that would allow this kind of
application, as was proposed in the thread I linked above.

I have considered using Evdev passthrough, but I find it very unreliable (I
got it to work for a mouse only via libvirt, keyboard works fine), but I
just couldn't get it to work for the touchpad: with plain QEMU the touchpad
kinda works but not as you'd expect, as the mouse teleports to where I
touch on the touchpad (as if there were a 1:1 mapping between the touchpad
and the screen), it's very weird and very not usable; and with libvirt I
couldn't get any mouse movement. But even if I could get it to work, it is
my understanding that Evdev passthrough just emulates a PS2 mouse, so that
means I'd lose any multitouch feature, which is not ideal.

Anyway, my laptop is running a 9th gen CoffeeLake Refresh H-series CPU on
Manjaro with Linux 5.3 as the main system.

Thank you very much,

Davide.
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