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

To: "Marek Marczykowski-Górecki" <marmarek@xxxxxxxxxxxxxxxxxxxxxx>

From: "Thierry Escande" <thierry.escande@xxxxxxxxxx>

Date: Thu, 09 Apr 2026 15:16:20 +0000

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Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx, "Anthony PERARD" <anthony.perard@xxxxxxxxxx>, "Juergen Gross" <jgross@xxxxxxxx>, "Daniel P. Berrangé" <berrange@xxxxxxxxxx>

Delivery-date: Thu, 09 Apr 2026 15:16:24 +0000

Feedback-id: 30504962:30504962.20260409:md

List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 4/9/26 12:16, Marek Marczykowski-Górecki wrote: > On Wed, Apr 08, 2026 at 05:23:39PM +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. > > Generally I like the approach, and it looks sensible on the first look. > But CI says it fails to build: > https://gitlab.com/xen-project/people/marmarek/xen/-/pipelines/2439831766 > I completely skipped YAJL support. My bad. I'll send a v2 pretty soon. Regards, Thierry -- Thierry Escande | Vates XCP-ng Developer XCP-ng & Xen Orchestra - Vates solutions web: https://vates.tech

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