Re: [Qemu-devel] [PATCH 3/9] arm: add missing v7 cp15 registers

[Peter Maydell <peter.maydell@linaro.org>]

On 22 December 2011 10:14, Avi Kivity <avi@redhat.com> wrote:
> On 12/21/2011 11:10 PM, Peter Maydell wrote:
>> Avi: is there a way for a device (sysbus device in this case) to
>> find out for one of its memory regions where it has been mapped
>> in the address space?
>
> Where and if.
>
>> The context here is that the Cortex-A9
>> has a cp15 register whose value is "base address of the private
>> peripherals", and it would be nice not to have to have boards
>> saying both "map mmio region at X" and "set property so register
>> reads as X"... [You could argue that hardware implementations
>> have to do the equivalent of both of these things separately,
>> I suppose, but it's still not very pretty.]
>
> I don't really follow, can you explain?

So in real hardware, these devices (interrupt controller,
timers, etc) are an integral part of the CPU. They appear in
the memory map at an address which is configured by hardwiring
the CPU's PERIPHBASE signals to specify that address. Since
obviously software needs to know where the devices are, there
is a coprocessor register which simply returns the value
of PERIPHBASE. (So I was wrong that hardware does the mapping
and the register separately -- sorry.)

Part of the problem we have is that in QEMU we don't model
the CPU as a single qdev device which includes both the
core and its builtin devices -- the two are completely
separate and the board model ends up having to do a lot of
the work of wiring things up, and in cases like this where
one bit of config affects both the core CPU and the builtin
devices you end up having to specify it twice.

> Anyway, with the new MemoryListener API (not yet merged), you can
> register a callback to be called when MemoryRegions become visible or
> invisible.  You can filter there for your pet region and get all the
> info about it.

Mmm, that would work.

-- PMM