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

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Mar 24, 2026 at 09:28:00AM +0000, Anthony PERARD wrote:
> 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.

-device accepts JSON directly, so there's no transformation you
need to do from the QMP JSON. For example:

  -device '{"driver":"pcie-root-port","port":8,"chassis":1,"id":"pci.1","bus":"pcie.0","multifunction":true,"addr":"0x1"}'

Likewise  -object, -audiodev and -netdev accept QMP JSON too

With regards,
Daniel
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