Re: [RFC PATCH v4 0/6] xl: Allow PCI devices to be passthrough'd via Qemu command line

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Thu, Apr 23, 2026 at 04:46:10PM +0200, Thierry Escande wrote:
> This series adds new 'hotplug' option for PCI device passthrough.
> 
> 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 them directly to the Qemu command line using xen-pci-passthrough
> device.

Hi Thierry,

I don't like this new "hotplug" option. It is just an implementation
detail on how the device is added to QEMU. It is just going to be
confusing for users. Because in both case, the device is "cold-plugged",
it just happen to be inserted after QEMU thing it is fully configured,
but the guest hasn't really started yet.

It is likely that we implemented the pci passthrough this way because it
worked and because we could reuse the same code path for real hotplug as
well.

So, for the patch that is really needed (adding the device on the
command line), we could do just for the Q35 machine, or we could do that
for all guests. For the second option, probably the best one, but we are
going to try to find out if everything work as expected, especially
Linux stubdomain.

> 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_qapi_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

I though you were renaming a public API function, but the original one
was static with the wrong name. (function prefixed with "libxl_", one
underscore, are in the public API, private one to libxl are prefixed
with "libxl__".)

> made available from libxl.h for access in libxl_dm.c. 2 other functions are

If you only mean to use a function internally, add it to
libxl_internal.h, I don't want to expose more functions to the API
without a good reason. `xl pci-assignable-list` already work, so I don't
think exposing a new function is useful.

Cheers,


--
Anthony Perard | Vates XCP-ng Developer

XCP-ng & Xen Orchestra - Vates solutions

web: https://vates.tech