Re: [Qemu-devel] [RFC ppc-next PATCH 3/6] memory: add memory_region_to_address()

[Peter Maydell]

On 21 March 2013 08:31, Alexander Graf <address@hidden> wrote:
> On 14.02.2013, at 07:31, Scott Wood wrote:
>> This is useful for when a user of the memory region API needs to
>> communicate the absolute bus address to something outside QEMU
>> (in particular, KVM).
>>
>> Signed-off-by: Scott Wood <address@hidden>
>
> Peter, how does the VGIC implementation handle this?

Check kvm_arm_register_device() in target-arm/kvm.c. Basically
the VGIC device model calls this function to say "tell the kernel
where this MemoryRegion is in the system address space, when it
eventually gets mapped". The code in kvm.c uses the memory system's
Notifier API to get a callback when the region is mapped into
an address space, which it uses to track the offset in the
address space. Finally, we use a machine init notifier so that
just before everything finally starts we can make the KVM ioctls
to say "here is where everything lives".

I think this is a pretty neat way of doing it because it means
neither the interrupt controller device nor the board model
really need to care about the kernel being told where things
are mapped; it's all abstracted out into kvm.c. If your
interrupt controller can be moved around at runtime that's
probably also handlable, but the ARM code just unregisters its
notifiers at machine init because the GIC can't move.

(I think the code assumes the device only gets mapped into
one address space; this could easily be fixed if it's not true
at some point in the future.)

thanks
-- PMM