[v1] xl: Allow PCI devices to be passthrough'd via Qemu command line

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

Posted by Thierry Escande 4 days, 3 hours ago

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/

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         | 31 ++++++++++++++++++++++
 tools/libs/light/libxl_internal.h   |  2 ++
 tools/libs/light/libxl_pci.c        | 41 ++++++++++++++++++++++-------
 tools/libs/light/libxl_types.idl    |  1 +
 tools/libs/util/libxlu_pci.c        |  2 ++
 tools/xl/xl_parse.c                 |  5 ++++
 9 files changed, 96 insertions(+), 10 deletions(-)

-- 
2.53.0



--
Thierry Escande | Vates XCP-ng Developer

XCP-ng & Xen Orchestra - Vates solutions

web: https://vates.tech

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

Posted by Marek Marczykowski-Górecki 3 days, 10 hours ago

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

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

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

Posted by Thierry Escande 3 days, 5 hours ago


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