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Re: [PATCH 0/9] Deprecate sysbus_get_default() and get_system_memory() e

From: Mark Cave-Ayland
Subject: Re: [PATCH 0/9] Deprecate sysbus_get_default() and get_system_memory() et. al
Date: Tue, 20 Sep 2022 16:36:26 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.13.0

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

On Tue, 20 Sept 2022 at 00:18, Bernhard Beschow <shentey@gmail.com> 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.



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