[vfio-users] Question about integrated GPU passthrough and initialization

[James Courtier-Dutton]

Hi,

I have had fun recently trying to get vega gpu passthru working.
After a lot of testing and some kernel patching I would say:
1) if you want vega gpu reset working, use kernel 5.1.5 or above in the
guest as well as the host.
2) if you want passthru working instead of a blank screen at vm start, use
OVMF efi instead of seabios.
3) you may or may not need a vfio patch that stops the graphics card
resizing the BAR.

Kind regards

James

On Mon, 3 Jun 2019, 22:40 Micah Morton, <mortonm at chromium.org> wrote:

> Hi Alex,
>
> Could you remind me whether there is a minimum recommended kernel
> version to be running in the VM guest when doing GPU passthrough?
>
> I'm fine running 4.14 in the host, but was looking to see if I could
> run 4.4 in the guest and couldn't remember if it is advised to use a
> newer kernel in the guest or if there is any reason to have the guest
> kernel match the host kernel version?
>
> On Wed, May 29, 2019 at 10:00 AM Micah Morton <mortonm at chromium.org>
> wrote:
> >
> > Ah my bad. Just realized I was using my own copy of SeaBIOS that I had
> > built. When I use the copy from qemu-3.0.0/pc-bios/bios-256k.bin I see
> > the i915 driver finding the OpRegion:
> > [    0.269341] in i915_driver_init_hw
> > [    0.269374] [drm] Memory usable by graphics device = 4096M
> > [    0.269585] in intel_opregion_setup
> > [    0.269600] graphic opregion physical addr: 0x7fffe000
> >
> > Still working on getting the screen to light up
> >
> > On Wed, May 29, 2019 at 9:44 AM Alex Williamson
> > <alex.williamson at redhat.com> wrote:
> > >
> > > On Wed, 29 May 2019 09:25:59 -0700
> > > Micah Morton <mortonm at chromium.org> wrote:
> > >
> > > > So as I mentioned, the ChromeOS firmware writes the location of the
> > > > OpRegion to the ASLS PCI config register
> > > > (
> https://github.com/coreboot/coreboot/blob/master/src/drivers/intel/gma/opregion.c#L88
> ).
> > > > The i915 driver then gets the address for the OpRegion from that
> > > > register here:
> https://elixir.bootlin.com/linux/latest/source/drivers/gpu/drm/i915/intel_opregion.c#L910
> .
> > > > This all works for Chrome OS, but when we run a VM with SeaBIOS the
> > > > ASLS PCI config register doesn't get written with the location of the
> > > > OpRegion.:
> > > > [    0.263640] in i915_driver_init_hw (I added this)
> > > > ...
> > > > [    0.263922] in intel_opregion_setup (and this)
> > > > [    0.263954] graphic opregion physical addr: 0x0 <-- This is
> > > > supposed to point to the OpRegion, not be zero.
> > > > [    0.263954] ACPI OpRegion not supported!
> > > > ...
> > > > [    0.267727] Failed to find VBIOS tables (VBT)
> > > >
> > > > I'm also not sure if the OpRegion is actually in VM memory or not. Do
> > > > you think I need to find a way to put the OpRegion in VM memory as we
> > > > have seen coreboot (Chrome OS firmware) do above? Or should using
> > > > "x-igd-opregion=on" somehow ensure that the OpRegion makes it into VM
> > > > memory? Clearly I at least need to find a way to set that ASLS PCI
> > > > config register in the VM or modify the i915 driver that runs in the
> > > > guest so it can find the OpRegion.
> > >
> > > In QEMU, vfio_pci_igd_opregion_init() adds the opregion to a fw_cfg
> > > file "etc/igd-opregion" and makes the (virtual) ASLS register
> > > writable.  Then in SeaBIOS, any Intel vendor ID, PCI class VGA device
> > > will trigger the intel_igd_setup() function, which looks for the fw_cfg
> > > file, allocates space for it, and writes the GPA back to the ASLS
> > > register.  That's at least how it's supposed to work, which again
> > > reminds me for the umpteenth time that x-igd-opregion only works with
> > > SeaBIOS as OVMF has rejected this support in favor of an option ROM
> > > based solution, which Intel never provided.  I think you're using
> > > SeaBIOS though so, so as long as that's not an ancient version it
> > > should do the little dance here.  The ASLS is writable though, we don't
> > > do any write-once tricks, so something could blindly stomp on it.  You
> > > might enable logging in SeaBIOS, it will emit some spew for the
> > > OpRegion support.  You could also enable tracing to see the write of
> > > the ASLS into QEMU.  Thanks,
> > >
> > > Alex
>
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