PCIe Pass-through lanes and lane performance
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Before you go further down the bridge tree, redakula called this one early in the thread and it's worth rereading.
Intel Arc cards put an internal PCIe bridge between the slot and the GPU, and
lspcion Linux only reports the top of that chain. His own A310 shows PCIe x8 4.0 at x4 4.0 in GPU-Z under Windows while Linux insists it is x1. That is the same bridge topology you're now staring at on 81, 82 and 83.TeddyAstie said much the same when he called it mostly a display issue, and he later checked that resizable BAR does work on UEFI guests since the OVMF build supports it. So the Gen1x1 reading may just be
lspcidescribing the bridge rather than the card. I could easily be wrong, and the people already in this thread know this hardware far better than I do.
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I'm in agreement with all of this. I can clearly see the Intel card starting at slot 81 and ending at 84. Unfortunately, in XCP-ng 8.3, only 83 (the "gpu" itself) is visible as an assignable PCI pass-through device. When I pass through just slot 83, I get a dysfunctional result.
Again...I'm a little out of my expertise here, but my assumption here is that in order to actually get the full GPU to work, I need to get slots 81-84 passed over to the VM.
Problem...In XCP-ng 8.2, you could mask off PCI devices with the pciback.hide function:
/opt/xensource/libexec/xen-cmdline --set-dom0 "xen-pciback.hide=(0000:04:01.0)(0000:00:19.0)" Or in my case: /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)"Then you would assign those same PCI devices to the guest VM.
In 8.3 you do this with an xe command:
xe pci-list and xe pci-disable-dom0-access uuid=<pci uuid>The bridge devices for the Intel card aren't being included in the pci-list of XCP-ng and are therefore not assignable to guests. For example, running "xe pci-list" on a host shows me this single entry relevant to the Intel card:
uuid ( RO) : f3e6842b-ad1b-9e93-dbea-214dde28618a vendor-name ( RO): Intel Corporation device-name ( RO): Device e212 pci-id ( RO): 0000:83:00.0With the new 8.3 method of assigning PCI resources to guest VM's, I don't see how I can fully pass-through the Intel GPU to a guest. I won't necessarily call this a "bug" but it is an unexpected and perhaps edge case behavior that Vates didn't expect.
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Also a question for @Team-XAPI-Network
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@JamesG You can still manually use
xen-cmdlineto pass through 81-84 to the VM on 8.3 (this is in fact whatxe pci-disable-dom0-accessdoes behind the scenes).I'll try to see why other devices are omitted and how this can be fixed.
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@JamesG Could you pass through 81-84 manually and confirm that it actually fixes the issues for you?
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@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.
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@JamesG it should be compatible, yes.
you can check the current command-line value with
/opt/xensource/libexec/xen-cmdline --get-dom0 xen-pciback.hideand add all 81-84 devices with--set-dom0 -
@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 loadedNot 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.
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Nevermind...Typo...Rebooting now to see...
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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 e2f7Execute 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
0000:82:02.0Looks 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.
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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.
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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 ?
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@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. -
@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. -
@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-accessdoes behind the scenes. What issues did you see? Which steps did you follow? -
@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
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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.
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@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.
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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.
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@JamesG Don't bother, I'm in PTO next week; so I may get access to hardware when I'm back.
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