Re: Intel IGD passthrough working intermittently

[Corvin Köhne]

On Wed, 2024-10-02 at 14:59 +0200, Marco wrote:
> CAUTION: External Email!!
> On Wed, Oct 2, 2024 at 2:30 PM Corvin Köhne <C.Koehne@beckhoff.com>
> wrote:
> > 
> > First of all, you should make sure to pass the correct gms value to
> > the
> > guest. I've already seen issues when passing the wrong value to
> > guests.
> > That's why I've recently send a patch to qemu to always reuse the
> > host
> > value [1].
> 
> Ok thanks, I seem to understand I should use the same value that the
> host uses, but how do I figure out that value?
> 

It's contained in bits 15:8 of PCI register 0x50. So, just take a look
into lspci -xxx.

Btw. if you compile latest qemu from source, it should pick the host
value by itself, regardless of the x-gms parameter passed.

> > I assume that you're booting in UEFI mode by using OVMF?
> 
> I'm booting using SeaBios, as my understanding from reading various
> sources (vfio mailing list and forums) is that it wouldn't work at all
> with OVMF (in fact I even think I tried EFI at the beginning, and
> never got any output).
> I might try again, just in case.
> One thing I didn't mention: I've tried using various machine types
> from pc-q35-6.2 up to pc-q35-8.2 without any noticeable change.
> 

I'm succesfully using IGD passthrough coupled with OVMF for years on the
bhyve hypervisor. I've recently started to look at qemu and want to use
it on qemu too. I don't have much experience with qemu yet but as far as
I can see, it should work from qemu side for Coffee Lake and prior CPUs.
I'm currently working on support for later CPUs.

As mentioned, it's just not supported by OVMF. Therefore, it requires a
patched OVMF. If you're interested in it, I can share my patches for
OVMF.

> > I don't know why it boots sometimes fine and sometimes not. However,
> > your black screen at boot is caused by a missing GOP. It's the EFI
> > graphics driver. That's why you only get graphics output when an OS
> > driver like i915 or the Windows driver is available. Unfortunately,
> > there're currently some constraints making it very hard to get a
> > working
> > GOP:
> 
> That's fine for my use case. I don't mind if I cannot see the initial
> part of the boot, as long as the windows part works reliably (which it
> doesn't at the moment, as explained).
> 
> Thanks!
> 
> 

Even though it's fine for your use case, it could be nice to have to get
some hints what might be wrong.

seabios should support graphics output too if you pass a VBIOS as
romfile to the guest. However, I don't have any experience with that. I
do not even know whether Intel has a VBIOS for Coffee Lake at all.


-- 
Kind regards,
Corvin

signature.asc
Description: This is a digitally signed message part