PCIe Pass-through lanes and lane performance

olivierlambert

andriy.sultanov

@JamesG You can still manually use xen-cmdline to pass through 81-84 to the VM on 8.3 (this is in fact what xe pci-disable-dom0-access does behind the scenes).

I'll try to see why other devices are omitted and how this can be fixed.

andriy.sultanov

@JamesG Could you pass through 81-84 manually and confirm that it actually fixes the issues for you?

JamesG

@andriy.sultanov Absolutely. I wasn't sure if the old way was incompatible with the new way or not.

So the old 8.2 method will still work without conflict in 8.3??

If so, I'll have an answer for you shortly.

andriy.sultanov

@JamesG it should be compatible, yes.

you can check the current command-line value with /opt/xensource/libexec/xen-cmdline --get-dom0 xen-pciback.hide and add all 81-84 devices with --set-dom0

JamesG

@andriy.sultanov First problem...

/opt/xensource/libexec/xen-cmdline --get-dom0 xen-pciback.hide
xen-pciback.hide=(0000:81:00.0)(0000:82.01.0)(0000:82:02.0)(0000:83:00.0)(0000:84:00.0)

That looks good...

xl pci-assignable-list
libxl: error: libxl_pci.c:444:libxl_device_pci_assignable_list: Looks like pciback driver not loaded

Not so much...

I did go ahead and assign those PCI devices to the guest, but of course it won't start, complaining that the pciback driver isn't loaded.

JamesG

Nevermind...Typo...Rebooting now to see...

JamesG

Argh...

Okay...First up:

lspci:

81:00.0 PCI bridge: Intel Corporation Device e2ff (rev 01)
82:01.0 PCI bridge: Intel Corporation Device e2f0
82:02.0 PCI bridge: Intel Corporation Device e2f1
83:00.0 VGA compatible controller: Intel Corporation Device e212
84:00.0 Audio device: Intel Corporation Device e2f7

Execute the hide:

/opt/xensource/libexec/xen-cmdline --set-dom0 "xen-pciback.hide=(0000:81:00.0)(0000:82:01.0)(0000:82:02.0)(0000:83:00.0)(0000:84:00.0)"

Reboot and verify the hide:

xl pci-assignable-list
0000:84:00.0
0000:82:01.0
0000:83:00.0
0000:81:00.0
0000:82:02.0

Looks good....make the assignment:

xe vm-param-set other-config:pci=0/0000:81:00.0,0/0000:82:01.0,0/0000:82:02.0,0/0000:83:00.0,0/0000:84:00.0 uuid=9d76f849-16bf-0414-9072-6904176bbb01

Verify:

xe vm-param-list uuid=9d76f849-16bf-0414-9072-6904176bbb01

other-config (MRW): pci: 0/0000:81:00.0,0/0000:82:01.0,0/0000:82:02.0,0/0000:83:00.0,0/0000:84:00.0; import_task: OpaqueRef:9e15d1aa-5e79-79d9-af81-4dc299725217; xo:9d76f849: {"creation":{"date":"2025-12-16T19:19:14.724Z","template":"1f8728f9-6354-4a68-adc3-222c34f2bb94","user":"ae9cf9c9-bbcc-4dc9-b1bb-d9fb1b657c60"}}; base_template_name: Windows 11; mac_seed: 5891089b-e9ec-3050-30e5-564db3e6e170; install-methods: cdrom

Starting up the VM results in this:

  "params": [
    "xenopsd internal error: Cannot_add(0000:82:02.0, Device_common.QMP_Error(1, \"{\\\"error\\\":{\\\"class\\\":\\\"GenericError\\\",\\\"desc\\\":\\\"Failed to initialize 11/15, type = 0x1, rc: -1\\\",\\\"data\\\":{}},\\\"id\\\":\\\"qmp-000006-1\\\"}\"))"
  ],

I'm gonna strip it all back, reboot and start over.

JamesG

Yep...

Stripped it all out....Rebooted...

Set the pcihideback for dom0...Rebooted.

Verified all devices were assignable.

Assigned to guest VM, booted...No go.

Initially the log said it couldn't assign 82:02.0
So I removed it.
Then it couldn't assign 82:01.0
So I removed it.
Then it couldn't assign 81:00.0
So I removed it.

Now the VM works...But we're effectively back to just passing through the GPU itself, and I guess the audio device.

No functional change.

TeddyAstie

@JamesG

You can't PCI Passthrough a PCI bridge (and even if you could, it wouldn't be very useful), as it's a part of PCI internals and not really something the guest requires to use the device.

As I asked previously, is there any issue (aside display like PCIe 1.0 1x) with the device, is it behavior correctly or having the expected performance ?

JamesG

@TeddyAstie From what I've tried...It won't work at all in Linux.

It kind of works in Windows, and while I do see some load on the GPU when running something, there's as much if not more load on the CPU. It's not what I expected at least.

I've never really gamed on PC, and definitely never Linux. My only uses for GPU's have been in media composition which was always on dedicated systems and Nvidia GPUs. More recently I've been doing some AI tasks, again with passed-through NVidia GPUs.

Side tangent...
I ran into another workload last night where the app wanted OpenGL...But OpenGL doesn't work with the "normal" VGA adapter in XCP-ng, and I don't have a GPU on that particular pool to try a pass-through. That lead me to VirGL, but it doesn't look like that's gone anywhere.

pandusen

@andriy.sultanov said:
I'll try to see why other devices are omitted and how this can be fixed.

Any news on this?
I have been fighting with getting an Intel Arc Pro B50 to work on Linux for months and i suspect that, the fact that the audio device isn't present, is what makes Linux fail to load the driver correctly.

Going the xen-cmdline way, seams to disable xe passthrough both in command line and in XO gui, causing all kinds of issues.

Windows seems to be able to initialise the GPU driver without the audio device, but it is in some kind of borked state. You can send workload to the GPU but the drivers are complaining about "no-pci compatibility".

XCP-ng 8.3 (latest patches)
Fedora server 44 - Kernel 7.1.5-201
on latest Arc firmware as well.

andriy.sultanov

@pandusen As Teddy said above, you can't passthrough a PCI bridge, so there's no PCI devices xapi shouldn't omit here.

But "going the xen-cmdline way" shouldn't break anything, that's what xe pci-disable-dom0-access does behind the scenes. What issues did you see? Which steps did you follow?

JamesG

@andriy.sultanov It's possible that this is just a fundamental issue with trying to use (maybe just Battlemage) Intel GPU's in XCP-ng.

The card itself appears to be a PCIe bridge in front of the actual GPU hardware on the card. If the driver on the guest requires "the whole thing" sent to it, and you can't pass-through the whole bridge and everything behind it, then there's no real way to use these GPU's in a guest on XCP-ng.

The lack of recent kernel or sufficient backports in the XCP-ng kernel, means the SRIOV features of Intel GPU's are lost as well.

All of this stuff works in Proxmox with seemingly little issues.

James

TeddyAstie

FYI I will get my hands on a Intel Arc Pro B50 to make various tests with it (in the next weeks). If something don't behave as expected, I will try to find a solution.

JamesG

@TeddyAstie I think there's alot of interest in the Intel platform because of the lack of ridiculous licensing for virtualization features. The problem is that you need a pretty current 6.x kernel to use them and XCP-ng/XenServer is just stuck on a really aging kernel right now.

As exposed in this thread, the PCIe pass-through isn't really working properly under XCP-ng.

Even if pass-through worked, that's kinda lame because the real purpose would be to chop up the GPU into SR-IOV instances so multiple workloads could have a GPU.

The Proxmox community is already rockin' and rollin' with this. I watched a video a couple of days ago where a B70 was chopped up into Qty.4, 8GB GPU's via SR-IOV and passed to clients.

No ridiculous mega-$$$$ licensing. It just worked.

JamesG

@TeddyAstie

Thought...If you want access to a server with a B50, I think I could setup VPN access and put a host online with IPMI access for you to do whatever you want with.

You will have an AMD Epyc 7302 with 64GB of ram some flash and HDD storage, and an Intel B50. PM me and we can work out the details of what you need to experiment with, and I can setup some remote access for you to get into the system.

I will gladly work with you guys any which way I can to move this forward.

TeddyAstie

@JamesG Don't bother, I'm in PTO next week; so I may get access to hardware when I'm back.

pandusen

@andriy.sultanov

@andriy.sultanov said:

@pandusen As Teddy said above, you can't passthrough a PCI bridge, so there's no PCI devices xapi shouldn't omit here.

I am not trying to pass through the bridge only the end points. The Intel arc's have 2 end points: The GPU and the Sound device. "xe pci-list" only reveals the GPU, not the sound device. (this works for nvidia and AMD)

But "going the xen-cmdline way" shouldn't break anything, that's what xe pci-disable-dom0-access does behind the scenes. What issues did you see? Which steps did you follow?

the sound device is available in the lspci list and can be passed through using CLI.
But doing so, (using CLI for passtrough) undoes everything done using xe or the passthrough gui in XO. and results in this:
https://xcp-ng.org/forum/topic/10609/xcp-ng-8.3-pci-passthrough-issue
so yes, its does break something.