Gotcha
Will do
Thanks for the help!

I've been trying to do pci passthrough of an AMD 5600xt . I got it to work on a Windows VM (though I did not test its performance), but now I want to assign the pci back to dom0, and I can't seem to find a straight answer on the web.

I tried

# xl pci-assignable-remove -r <uuid>

based on xen documentation but that did not work

Out of frustration I did:

/opt/xensource/libexec/xen-cmdline --set-dom0 "xen-pciback.hide=()"

and when I checked

pci-assignable-list

I got

libxl: error: libxl_pci.c:427:libxl_device_pci_assignable_list: Looks like pciback driver not loaded

Does any one see this being an issue/problem later on?

What would be the proper command to give the pci back to dom0?

@toby4213
Oh I see. I may have read incorrectly. I honestly would not be able to tell you if that is the reason why you are having issues. I do know, however, that the 3.5GB of ram error is fairly common for people using Xen and wanting to pass-though a graphics card. If what you send is linked to that error, then it might be related.

@toby4213
I don't think since it seems from the link you posted that the latest Xen version resolve the issue.
TBH most of this goes way above my head...

I haven't successfully passed-through a graphics card with XCP-ng. The closest I have gotten was to pass-through the card and install the drivers, but it would not display anything. This was done with a previous version of XCP-ng, so that might be an avenue to take. I wasn't even able to install the drivers in the latest XCP-ng version.

I have, however, have had success with Qubes OS, so this might not apply 1 to 1.

Basically there is an issue with Xen which does not allow VMs with a graphics card passed-through to boot correctly when they are assigned more than 3.5GBs of ram. There is a work around for Qubes, which gets passed this.

https://neowutran.ovh/qubes/articles/gaming_windows_hvm.html

You have to make sure that the graphics card is blacklisted at GRUB, so you shouldn't be able to see the XCP-ng console if you do it correctly, as you said.

Another problem that I've run into was installing the drivers. Basically, as soon as you are able after you've installed Windows, you should download your drivers, turn off the vm, and remove the network connection to the vm. Then, reboot the vm and install the drivers. Having the vm connected to the internet, for some reason would not allow the drivers to install correctly. The graphics card worked correctly after that.

I did run into a problem similar to yours when I passed-through my motherboard's usb's, however. With a 5600xt everything worked perfectly, but with a gtx 1070, the vm became unusable after passing-though the usb's, so this might be an issue with nvidia.

Since the issue is with Xen you might be able to duplicate the process above in XCP-ng.

Keep us posted if it works.

@lightspeed
I don't have your problem exactly, but while I didn't have any issues passingthrough a consumer grade graphics card in 8.1 I have had issues in 8.2
For some reason, the Windows vm I passthrough the graphics card to reads it as a usb device.

I've never gotten errors like that, it just doesn't work when it is pass through.
Another thing I found a little odd was that the graphics card shows up as a usb device in Windows

@johnkrips

/opt/xensource/libexec/xen-cmdline --delete-dom0 xen-pciback.hide

https://xcp-ng.org/docs/compute.html#pci-passthrough

@jmccoy555
It's definite not the same for consumer grade cards unfortunately
At least not any recent card. I have an old ATI HD5xxx card that XCP-ng recognizes and am able to assign to a VM via the VM's advance settings in XO.

Do you need to install the drivers for that card in the VM, or does it just work?

The problem for me arises when I try to install the drivers, but before I try to install them the VM becomes very unstable, which tells me there's something going wrong when I pass the pci lanes

I have been trying to pass through a graphics card as well.
Previously, on XCP-ng 8.1, I had no issues doing this, except for the reset issue that is caused by the graphics card. On XCP-ng 8.2, passing through the graphics card has become really difficult. The VM that is assigned the pci lanes, in my situation a Windows vm, becomes really unstable. The vm sees the graphics card, but if I attempt to install the Radeon drivers, the vm crashes and I'm no longer able to start it.
@Johnkrips These issues might be due to the pci issues related to the 8.2
https://xcp-ng.org/forum/topic/4033/persistent-usb-passthrough?_=1610136013449
https://github.com/xcp-ng/xcp/issues/470

olivierlambert created this issue in xcp-ng/xcp

closed PCI passthrough issues #470

sure
msi x570 unify
xfx 5600xt
ryzen 3900xt

@technot
This is interesting. I'm able to download the latest amd drivers and install them, only issue being what I've mentioned before about getting error 43 until I reboot the server.