Re: [RFC PATCH v2 0/3] xl: Allow PCI devices to be passthrough'd via Qemu command line

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Thu, Apr 09, 2026 at 04:20:57PM +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, the function libxl_pci_assignable() is made available from libxl.h
> for access in libxl_dm.c. I don't know if it's ok to make it public or if
> it should stay private. Maybe it should be renamed as
> libxl_device_pci_assignable() to match the other PCI device APIs.
> 
> 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@vates.tech/

I don't see relevant device option added to the QEMU cmdline:
https://gitlab.com/xen-project/people/marmarek/xen/-/jobs/13860278916

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

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