Re: [Qemu-devel] [RFC 1/4] [UNTESTED] xen: Don't force has_dynamic_sysbus on machine class

[Eduardo Habkost]

On Fri, Mar 24, 2017 at 11:24:31AM +0100, Juergen Gross wrote:
> On 24/03/17 11:09, Peter Maydell wrote:
> > On 24 March 2017 at 08:23, Juergen Gross <address@hidden> wrote:
> >> On 23/03/17 22:28, Eduardo Habkost wrote:
> >>> The xen-backend devices created by the Xen code are not supposed
> >>> to be treated as dynamic sysbus devices. This is an attempt to
> >>> change that and see what happens, but I couldn't test it because
> >>> I don't have a Xen host set up.
> >>>
> >>> If this patch breaks anything, this means we have a bug in
> >>> foreach_dynamic_sysbus_device(), which is supposed to return only
> >>> devices created using -device.
> >>>
> >>> The original code that sets has_dynamic_sysbus was added by
> >>> commit 3a6c9172ac5951e6dac2b3f6cbce3cfccdec5894, but I don't see
> >>> any comment explaining why it was necessary.
> >>
> >> xen-backend devices are created via qmp commands when attaching new
> >> pv-devices to a domain. They can be dynamically removed, too. Setting
> >> has_dynamic_sysbus was necessary to support this feature.
> > 
> > This seems like it ought to be handled by marking the xen-backend
> > devices as being ok-to-dynamically-create somehow, not by marking
> > the machine as supporting dynamic-sysbus (which it doesn't).
> > Maybe we don't have the necessary support code to do that though?
> 
> When writing the patches I couldn't find a way to do it differently.
> OTOH I'm not so deep in qemu internals I'd be able to add the needed
> support.
> 
> I'd be happy to test any patch, though.

If xen-backend devices are created via QMP commands, then
has_dynamic_sysbus is (currently) really needed, although I would
have preferred to set it on all x86 machines instead of changing
MachineClass::has_dynamic_sysbus outside class_init.

But with the new whitelist implemented by this series, we could
simply include xen-backend in the whitelist for the machines that
can be used with Xen, and get rid of xen_set_dynamic_sysbus().

I assume plugging/unplugging xen-backend devices apply to both
xen{pv,fv} and pc,accel=xen, right? Do we need to make it work
with "-machine none,accel=xen" and "-machine isapc" too?

-- 
Eduardo