Re: [PATCH 0/4] Xen: Add Q35 initial support for HVM guests

["Anthony PERARD" <anthony.perard@xxxxxxxxxx>]

On Sat, Mar 14, 2026 at 03:16:15PM +0100, Marek Marczykowski-Górecki wrote:
> On Fri, Mar 13, 2026 at 04:46:56PM +0000, Thierry Escande wrote:
> > This patch series introduces Q35 chipset support for Xen HVM guests. This is
> > based on the work from Alexey Gerasimenko (See patchset [1] for reference).
> >
> > This series does not implement device passthrough as this will need more 
> > work
> > on the Xen side to support multiple PCI buses. Since Q35 chipset does not
> > support device hotplugging and with current Xen mechanism for device
> > passthrough, they have to be plugged to a new PCIe root port, i.e. not the 
> > PCI
> > bus 0.
>
> I always found it weird that Xen does hot-plugging of PCI passthrough
> devices... Wouldn't it be easier to change this (connect them from the
> very start, via QEMU cmdline) and declare hotplug as unsupported (at
> least at this time)? I realize it's more complex change at the toolstack
> side, but overall may simplify things and maybe even improve
> reliability.

Skipping hot-plug on Q35 sounds good to me.

I though we could keep using QMP to do cold plug, with the experimental
"--preconfig", but "device_add" isn't allowed :-(. So yes, doing this
via command line sounds fine.

In libxl, we might be able to transform the json for `device_add` into
the command line counterpart for `-device` fairly easily so the argument
are only generated in a single place. Then we could skip the QMP
"device_add" in libxl when it's already done via command line.

But looks like XCP-ng also does QMP "device_add" for passthrough :-( but
one step-at a time.

Cheers,


--
Anthony Perard | Vates XCP-ng Developer

XCP-ng & Xen Orchestra - Vates solutions

web: https://vates.tech