• Hi.

    I wonder, is it possible to set machine type q35 for a windows vm somehow?

  • Machine type? Are you talking about QEMU stuff? If so that is only for emulation, XCP-ng is virtualization so you get the same hardware in your VM as the physical hardware on your host.

  • Im sorry tony, but that is not accurate. HVM guests do have emulated hardware.

    From Xen's website, source url underneath:

    Full Virtualization or Hardware-assisted virtualization (HVM) uses virtualization extensions from the host CPU to virtualize guests. HVM requires Intel VT or AMD-V hardware extensions. The Xen Project software uses Qemu to emulate PC hardware, including BIOS, IDE disk controller, VGA graphic adapter, USB controller, network adapter etc. Virtualization hardware extensions are used to boost performance of the emulation.


    Xen emulates i440 chipset as standard. I found some information in the xen mailinglist about work beeing done with q35 few years back. But it was not clear to me if that was ever finnished or howto enable it or not.

    here is a small snippit from the mailinglist:
    "Anyway, basic Q35 support can be added to Xen as an optional feature - Anthony
    suggested adding a 'machine=q35'config parameter which will allow enabling Q35
    support on demand while leaving the old machine by default. At this stage Q35
    support needs a lot of testing and collecting further bugs."

    source: https://lists.xenproject.org/archives/html/xen-devel/2017-06/msg03451.html

    This was back in 2017, but theres no documentation on howto set the machine type in either xenserver or xcp-ng.

  • XCP-ng Team

    Emulation is just needed on boot. Then, PV drivers are loaded and it works with those.

    You don't change the machine type or whatever else in Xen, you rely on PV drivers.

  • Maybe I misunderstand it all. But I do know PV drivers means less emulation is needed.
    However, the way i read it there are distinctions between HVM, PVHVM and HVM+PV drivers.
    Anyway, the PV drivers means you dont need emulation for disk and network IO. But chipset and its capabilities are still an emulated i440.
    As you can see from this summary, all three types uses software virtualized motherbord(chipset):

  • XCP-ng Team

    If you explain why you'd like to have another chipset at the first place, maybe we can move forward 😛

  • Basicly becouse some OVMF functionality requires q35.
    Im trying to increase stability and performance of GPU passthrough to a windows guest. And Ive read that for QEMU, choosing q35 emulation is better then i440 for this specific usecase (windows guest with gpu passthrough), wich is why i started looking into this in the first place. And thats how i came across the xen mailinglist where this was discussed back in 2017.

  • for instance, as you might remember from an older post, i needed to run an old amd driver and block windows from updating gpu drivers, in order for gpu to function.
    This behaviour is described by many qemu users as well, and by changing from from the default i440 to q35, they managed to use the newest amd drivers. Wich is what im trying to achive.

    I am currently not able to use a newer driver then Win10-64Bit-Radeon-Software-Adrenalin-Edition-18.4.1-Apr26.
    my previous post about this issue: https://xcp-ng.org/forum/topic/1887/solved-gpu-passthrough-to-linux-guest-works-fine-but-windows-guest-crash-on-driver-install?_=1599647598931

    I have since then upgraded xcp-ng from 7.6 to 8.2 in the hopes that would resolve the issue. But alas, it did not:) Still stuck with old driver.

  • XCP-ng Team

    After investigation, it seems that yet, it just doesn't work.

    However, for various technical reasons, it would be great to do so (even outside AMD GPU scope)

    I'm investigating on how to make things happen.

  • Awsome 🙂
    Im sure your investigation into this will be far superior to mine!

    Thanks for looking it up:)

  • XCP-ng Team

    Obviously, helping on the contribution would be great if you can. We'll probably start by asking on Xen devel.

  • I will continue to lookup information, and will happily test different things.

    But if it comes down to someone needing to write the code to emulate q35, then im sorry to say that is not within my abilities.

    Will update here if I make any progress on either q35 or the AMD driver issue.

  • @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.

  • @l1c May I ask, what amd gpu you are using, and maybe also what brand and make of motherboard/cpu?

  • sure
    msi x570 unify
    xfx 5600xt
    ryzen 3900xt

  • @l1c

    So there is a huge difference in our scenarios. AMD vs INTEL.
    IOMMU vs VT-d.

    It may be that my issue lies with the VT-d implementation on my MSI big bang xpower ii mainboard. Its quite old, so is the cpu.
    Its basicly from the time that VT-d functionality was just starting to make its way to consumer grade hardware, and not only 6k usd xeons with equallu expensive supermicro motherboards and alike.
    Not many needed this on consumer hardware at the time, and the mobo vendors cut many corners. The list of consumer mobos with broken VT-d implementations from then is quite extensive.
    Maybe I should just get a used old supermicro mobo from ebay instead. Or maybe its the VT-d implementation of the old i7-3930k cpu, and just picking up an old xeon from ebay would work better. I just like the idea of repurposing this now very old gaming rig to something usefull. Its still quite beefy tbh.

  • @olivierlambert
    I havent found any official documentation, but Ive been poking around a bit and so far I have found some information that seems interesting.

    It looks like xen allways starts vms using the quem.
    like this:
    65540 23960 1.5 0.2 261356 14740 ? SLl Sep11 11:09 qemu-dm-5 -machine pc-0.10,accel=xen,max-ram-below-4g=4026531840,allow-unassigned=true,trad_compat=true -vnc unix:/var/run/xen/vnc-5,lock-key-sync=off -monitor null -xen-domid 5 -m size=4096 -boot order=cdn -usb -device usb-tablet,port=2 -smp 4,maxcpus=4 -serial pty -display none -nodefaults -trace enable=xen_platform_log -sandbox on,obsolete=deny,elevateprivileges=allow,spawn=deny,resourcecontrol=deny -S -global PIIX4_PM.revision_id=0x1 -global ide-hd.ver=0.10.2 -global piix3-ide-xen.subvendor_id=0x5853 -global piix3-ide-xen.subsystem_id=0x0001 -global piix3-usb-uhci.subvendor_id=0x5853 -global piix3-usb-uhci.subsystem_id=0x0001 -global rtl8139.subvendor_id=0x5853 -global rtl8139.subsystem_id=0x0001 -parallel null -qmp unix:/var/run/xen/qmp-libxl-5,server,nowait -qmp unix:/var/run/xen/qmp-event-5,server,nowait -device xen-platform,addr=3,device-id=0x0001,revision=0x2,class-id=0x0100,subvendor_id=0x5853,subsystem_id=0x0001 -drive file=,if=ide,index=3,media=cdrom,force-lba=off -drive file=/dev/sm/backend/2c466dab-0424-67c4-3a1b-3257cab0cf54/695c548c-5600-4c9b-b785-5de42747b0a5,if=ide,index=0,media=disk,force-lba=on,format=raw -device rtl8139,netdev=tapnet0,mac=0a:c2:0e:56:dd:ba,addr=4 -netdev tap,id=tapnet0,fd=7 -device VGA,vgamem_mb=8,rombar=1,romfile=,subvendor_id=0x5853,subsystem_id=0x0001,addr=2,qemu-extended-regs=false -vnc-clipboard-socket-fd 4 -xen-domid-restrict -chroot /var/xen/qemu/root-5 -runas 65540.998

    I then looked around inside xcp-ng for different quem binarys and config files, and what struck me first is this:

    [root@localhost ~]# /usr/lib64/xen/bin/qemu-system-i386 -machine help
    Supported machines are:
    pc-i440fx-2.9 Standard PC (i440FX + PIIX, 1996)
    pc-i440fx-2.8 Standard PC (i440FX + PIIX, 1996)
    pc-i440fx-2.7 Standard PC (i440FX + PIIX, 1996)
    pc-i440fx-2.6 Standard PC (i440FX + PIIX, 1996)
    pc-i440fx-2.5 Standard PC (i440FX + PIIX, 1996)
    pc-i440fx-2.4 Standard PC (i440FX + PIIX, 1996)
    pc-i440fx-2.3 Standard PC (i440FX + PIIX, 1996)
    pc-i440fx-2.2 Standard PC (i440FX + PIIX, 1996)
    pc Standard PC (i440FX + PIIX, 1996) (alias of pc-i440fx-2.10)
    pc-i440fx-2.10 Standard PC (i440FX + PIIX, 1996) (default)
    pc-i440fx-2.1 Standard PC (i440FX + PIIX, 1996)
    pc-i440fx-2.0 Standard PC (i440FX + PIIX, 1996)
    pc-i440fx-1.7 Standard PC (i440FX + PIIX, 1996)
    pc-i440fx-1.6 Standard PC (i440FX + PIIX, 1996)
    pc-i440fx-1.5 Standard PC (i440FX + PIIX, 1996)
    pc-i440fx-1.4 Standard PC (i440FX + PIIX, 1996)
    pc-1.3 Standard PC (i440FX + PIIX, 1996)
    pc-1.2 Standard PC (i440FX + PIIX, 1996)
    pc-1.1 Standard PC (i440FX + PIIX, 1996)
    pc-1.0 Standard PC (i440FX + PIIX, 1996)
    pc-0.15 Standard PC (i440FX + PIIX, 1996)
    pc-0.14 Standard PC (i440FX + PIIX, 1996)
    pc-0.13 Standard PC (i440FX + PIIX, 1996)
    pc-0.12 Standard PC (i440FX + PIIX, 1996)
    pc-0.11 Standard PC (i440FX + PIIX, 1996)
    pc-0.10 Standard PC (i440FX + PIIX, 1996)
    pc-q35-2.9 Standard PC (Q35 + ICH9, 2009)
    pc-q35-2.8 Standard PC (Q35 + ICH9, 2009)
    pc-q35-2.7 Standard PC (Q35 + ICH9, 2009)
    pc-q35-2.6 Standard PC (Q35 + ICH9, 2009)
    pc-q35-2.5 Standard PC (Q35 + ICH9, 2009)
    pc-q35-2.4 Standard PC (Q35 + ICH9, 2009)
    q35 Standard PC (Q35 + ICH9, 2009) (alias of pc-q35-2.10)
    pc-q35-2.10 Standard PC (Q35 + ICH9, 2009)
    isapc ISA-only PC
    none empty machine
    xenfv Xen Fully-virtualized PC
    xenpv Xen Para-virtualized PC

    This is the extensive list of supported -machine arguments that the xens qem binary claims is supported. And as I suspected that i440 is the default. Its used as -machine pc-0.10 when its called by xen.
    I havent found anyway, yet, to make xen switch that -machine paramater. But I tried the most obvious wich was to add machine type as a key to the platform paramater inside a template like this:
    xe template-param-set uuid=552bce37-51b2-445d-84f2-5f33fa112d7e platform:machine=pc-q35-2.10

    verified that it was added:
    [root@localhost ~]# xe template-param-list uuid=552bce37-51b2-445d-84f2-5f33fa112d7e | grep platform
    platform (MRW): machine: pc-q35-2.10; hpet: true; nx: true; device-model: qemu-upstream-compat; pae: true; apic: true; viridian: true; acpi: 1

    Then made a vm using that template. It didnt do anything, quem was still spawned using -machine pc-0.10

    There is another paramater in the template that could be
    hardware-platform-version ( RO): 0
    But its readonly, so cant be changed using xe template-param-set

    Anyway, I dont know if the machine paramater used is hardcoded or not. There are also different quem binarys available:

    The one spawned by xen when launching a VM is the quem-dm or maybe the wrapper. And that binary does not have the same machine support list:
    [root@localhost ~]# /usr/libexec/xen/bin/qemu-dm -M help
    Supported machines are:
    xenfv Xen Fully-virtualized PC (default)
    xenpv Xen Para-virtualized PC

    Anyway, the fact that there is a quem binary claiming to support q35, i am getting my hopes up that it can infact be done. We allready know that the normal quem has the support. And I belive it may just be a matter of making a template that can choose wich binary and paramters to use.
    Maybe even add kernel support, i havent looked at the xcp-ng kernel source to see what virtualization is enabled or not.
    If I have time later this weekend, i will download a build-env docker and start to poke around the xcp-ng kernel and build one with any missing virtualizations missing.
    I was planning to do that anyway to see if i can figure out what changes was made in regards to my problem using any xcp-ng version above 7.6 with sucess.

    Maybe I will look at some of the other sources as well to see if I can find if the quem binary used is hardcoded, and the if maybe some of the paramaters passed to it, like -machine, is hardcoded as well.

    Thats it for now. Sorry for the long and messy post.

  • I havent spent very much time on this yet.
    And I have never looked at xen sources or dived into how it works. So please bare with me if my poking around will take some time.
    But for now, i dont really see any reason why xcp-ng shouldnt be able to run the normal quem, instead of the xen modified quem and connect to xcp-centers console with vnc, as it does with the xen modified quem.

    correct me if Im completly wrong, please:)

  • a couple ideas to try later is to just modify a template.json and reinstall the templates. Modifying only this part:
    "platform": {
    "viridian": "true",
    "device-model": "qemu-upstream-compat"

    to the q35.
    However, that alone will not be enough, next would be to
    modify this script to make it use the qemu-system-i386 binary: /usr/libexec/xenopsd/qemu-dm-wrapper
    Maybe make it look for the -m switch, and run the i386 binary only incase of -m q35, to keep it all running compatible with how it is now. That would only call the i386 binary if q35 switch is detected.

    But before I start wasting any time on this, is there any chance this could work?
    Or would I first need to enable anything virtualization-wise in the dom0 kernel or maybe even in the xen kernel?

  • @olivierlambert

    I took some time to test Q35 on qemu+kvm on ubuntu.
    Q35 is amazing. Not only can I now use the newest amd driver(20.9.2) as opposed to 18.4.1 that was the highest driver I could make work with xen/i440, but Q35 also seem to be handling FLR perfectly. Using i440 I would often have to shutdown dom0 to make the GPU reset and be able to use it in again in a vm after a vm reboot. With Q35 I have not had a single reset issue, and I have tried hard. I have rebooted the vm repeatedly both soft and hard, and not once has it locked up on reset.
    I also did a test with i440 on KVM to see if my issue with driver and reset bug was related to kvm/xen or the emulated chipset i440 vs q35. And the issues are the same with i440 on kvm, would not accept driver above 18.4.1 and hard reset of VM would cause gpu reset issue, only fixable by power cycling host(reboot is not enough. complete shutdown is needed).
    I therefore conclude the chipset is the key factor.
    I do have to wonder why xen is having a hardtime making Q35 work tho, concidering Xen basicly is/was using modified qemu. I am thinking making Q35 work for Xen has not been a priority. But it really should!
    This works so well, Ive decided to build a new desktop to use this.

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