Re: [Qemu-devel] q35 and sysbus devices

[Eduardo Habkost <ehabkost@redhat.com>]

On Fri, Mar 24, 2017 at 02:13:48PM +0000, Peter Maydell wrote:
> On 24 March 2017 at 13:48, Eduardo Habkost <ehabkost@redhat.com> wrote:
> > The problem with today's mechanism is that we have no way to make
> > a machine accept one type of sysbus device without making it
> > start accepting every other sysbus devices. If we thought all
> > !cannot_instantiate_with_device_add_yet sysbus devices were
> > already safe, we wouldn't have has_dynamic_sysbus in the first
> > place, would we?
> 
> Almost all sysbus devices are not dynamically instantiable
> and never will be [...]

Exactly. But most of them do _not_ have
cannot_instantiate_with_device_add_yet set. What I don't know is:
should they?

>              [...] because they need to map mmio and wire
> up IRQs and so on, and we have no mechanism for doing that.
> So the default position is "no, you can't do that (but maybe
> a subclass which knows better might override)". has_dynamic_sysbus
> is a dodgy hack for boards which support the "platform bus"
> which is a special case where the user can use -device
> for certain sysbus devices that the platform bus knows about
> and can wire up itself (in a range of irqs and memory that
> the board has set aside for that purpose). The default
> is still "no, you can't dynamically instantiate this", but
> instead of this (and the handful of exceptions) being imposed
> at compile time it's imposed (with exceptions) at runtime.
> 
> So for instance the ARM 'virt' board sets has_dynamic_sysbus,
> which allows a few devices to be added with -device, but
> most still do not permit it (they'll end up hitting the
> error_report() in hw/arm/sysbus-fdt.c:add_fdt_node()).

I sent a series yesterday that implements a generic mechanism for
boards to set the list of accepted device types, to replace
has_dynamic_sysbus. That part was easy. But I still don't know
what to do about q35.

-- 
Eduardo