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    PCIe Pass-through lanes and lane performance

    Scheduled Pinned Locked Moved Compute
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    • A Offline
      andriy.sultanov Vates 🪐 XAPI & Network Team @JamesG
      last edited by

      @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.

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      • A Offline
        andriy.sultanov Vates 🪐 XAPI & Network Team @JamesG
        last edited by

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

        J 1 Reply Last reply Reply Quote 0
        • J Offline
          JamesG @andriy.sultanov
          last edited by

          @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.

          A 1 Reply Last reply Reply Quote 0
          • A Offline
            andriy.sultanov Vates 🪐 XAPI & Network Team @JamesG
            last edited by

            @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

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            • J Offline
              JamesG @andriy.sultanov
              last edited by

              @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.

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              • J Offline
                JamesG @JamesG
                last edited by

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

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                • J Offline
                  JamesG @JamesG
                  last edited by

                  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.

                  J 1 Reply Last reply Reply Quote 0
                  • J Offline
                    JamesG @JamesG
                    last edited by

                    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.

                    TeddyAstieT 1 Reply Last reply Reply Quote 0
                    • TeddyAstieT Offline
                      TeddyAstie Vates 🪐 XCP-ng Team Xen Guru @JamesG
                      last edited by

                      @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 ?

                      J 1 Reply Last reply Reply Quote 0
                      • J Offline
                        JamesG @TeddyAstie
                        last edited by

                        @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.

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