Re: [RFC PATCH v3 0/3] xl: Allow PCI devices to be passthrough'd via Qemu command line

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Fri, Apr 10, 2026 at 03:03:45PM +0000, Thierry Escande wrote:
> This series adds support for a new 'hotplug' option for PCI devices to be
> passthrough'd to guests.
> 
> The current mechanism for device passthrough uses QMP device_add command
> to hot-plug PCI devices to the guest Qemu instance. This is an issue for
> guests running on Q35 chipset (preliminary support posted at [1]) since the
> Q35 PCI root bus does not support hotplug. Devices could be hotplugged to
> a secondary PCI bus but Xen only support 1 PCI bus for now.
> 
> The 'hotplug' option allows to control how devices are attached to the
> guest, either by using the legacy QMP mechanism (this is the default) or by
> passing it directly to the Qemu command line using xen-pci-passthrough
> device.
> 
> Example usage in cfg file:
>  pci = [ "00:03.0,seize=1,hotplug=0" ]
> 
> Since Qemu -device option accepts parameters in json format, this
> patchset adds a new internal function libxl__device_pci_get_qmp_json() that
> generates the device json object used for both QMP hotplug and command line
> passing.
> 
> Also, to handle the assignable PCI device list, the function
> libxl_pci_assignable() is renamed as libxl_device_pci_assignable() and is
> made available from libxl.h for access in libxl_dm.c. 2 other functions are
> also exported internally: libxl__pciback_dev_is_assigned() and
> libxl__device_pci_assignable_add().
> 
> This has been successfully tested on Xen 4.22-dev and Qemu 10.0 running
> Debian VMs in both Bios and UEFI mode with a passthrough'd nvme disk.
> Disabling the hotplug mechanism might be made mandatory for Q35 machines
> later, once Q35 support is merged upstream.
> 
> [1] 
> https://lore.kernel.org/xen-devel/20260313163455.790692-1-thierry.escande@xxxxxxxxxx/

Much better now:
https://gitlab.com/xen-project/people/marmarek/xen/-/pipelines/2444612680

I get now something like this:
libxl: debug: libxl_dm.c:3044:device_model_launch: Domain 1:  -device
libxl: debug: libxl_dm.c:3044:device_model_launch: Domain 1:  
{"driver":"xen-pci-passthrough","id":"pci-pt-03_00.0","hostaddr":"0000:03:00.0"}

So, it works :)

> Changes in v2:
> - Add support for YAJL json parser
> 
> Changes in v3:
> - Move code block of device command line parameters creation
> - Better handling of pci device assignation
> 
> Thierry Escande (3):
>   xl: Add pci device hotplug option
>   libxl: Allow PCI device passthrough using -device Qemu command line
>   docs: provide description for pci hotplug option
> 
>  docs/man/xl-pci-configuration.5.pod | 17 ++++++
>  docs/man/xl.cfg.5.pod.in            |  6 ++
>  tools/include/libxl.h               |  1 +
>  tools/libs/light/libxl_dm.c         | 85 +++++++++++++++++++++++++++++
>  tools/libs/light/libxl_internal.h   |  7 +++
>  tools/libs/light/libxl_pci.c        | 57 +++++++++++++------
>  tools/libs/light/libxl_types.idl    |  1 +
>  tools/libs/util/libxlu_pci.c        |  2 +
>  tools/xl/xl_parse.c                 |  5 ++
>  9 files changed, 163 insertions(+), 18 deletions(-)
> 
> -- 
> 2.53.0
> 
> 
> 
> --
> Thierry Escande | Vates XCP-ng Developer
> 
> XCP-ng & Xen Orchestra - Vates solutions
> 
> web: https://vates.tech
> 

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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