Re: [PATCH 0/9] Deprecate sysbus_get_default() and get_system_memory() et. al

[Mark Cave-Ayland <mark.cave-ayland@xxxxxxxxxxxx>]

On 20/09/2022 10:55, Peter Maydell wrote:

> On Tue, 20 Sept 2022 at 00:18, Bernhard Beschow <shentey@xxxxxxxxx> wrote:
> > In address-spaces.h it can be read that get_system_memory() and
> > get_system_io() are temporary interfaces which "should only be used temporarily
> > until a proper bus interface is available". This statement certainly extends to
> > the address_space_memory and address_space_io singletons.
> This is a long standing "we never really completed a cleanup"...
>
> > This series attempts
> > to stop further proliferation of their use by turning TYPE_SYSTEM_BUS into an
> > object-oriented, "proper bus interface" inspired by PCIBus.
> >
> > While at it, also the main_system_bus singleton is turned into an attribute of
> > MachineState. Together, this resolves five singletons in total, making the
> > ownership relations much more obvious which helps comprehension.
> ...but I don't think this is the direction we want to go.
> Overall the reason that the "system memory" and "system IO"
> singletons are weird is that in theory they should not be necessary
> at all -- board code should create devices and map them into an
> entirely arbitrary MemoryRegion or set of MemoryRegions corresponding
> to address space(s) for the CPU and for DMA-capable devices. But we
> keep them around because
>   (a) there is a ton of legacy code that assumes there's only one
>       address space in the system and this is it
>   (b) when modelling the kind of board where there really is only
>       one address space, having the 'system memory' global makes
>       the APIs for creating and connecting devices a lot simpler
>
> Retaining the whole-system singleton but shoving it into MachineState
> doesn't really change much, IMHO.
>
> More generally, sysbus is rather weird because it isn't really a
> bus. Every device in the system of TYPE_SYS_BUS_DEVICE is "on"
> the unique TYPE_SYSTEM_BUS bus, but that doesn't mean they're
> all in the same address space or that in real hardware they'd
> all be on the same bus. sysbus has essentially degraded into a
> hack for having devices get reset. I really really need to make
> some time to have another look at reset handling. If we get that
> right then I think it's probably possible to collapse the few
> things TYPE_SYS_BUS_DEVICE does that TYPE_DEVICE does not down
> into TYPE_DEVICE and get rid of sysbus altogether...
Following on from one of the discussion points from Alex's KVM Forum BoF session: I 
think longer term what we need to aim for is for QEMU machines to define their own 
address spaces, and then bind those address spaces containing memory-mapped devices 
to one or more CPUs.
Once this in place, as Peter notes above it just remains to solve the reset problem 
and then it becomes possible to eliminate sysbus altogether as everything else can 
already be managed by qdev/QOM.

ATB,

Mark.