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Re: [Qemu-ppc] [RFC ppc-next PATCH 3/6] memory: add memory_region_to_add

From: Peter Maydell
Subject: Re: [Qemu-ppc] [RFC ppc-next PATCH 3/6] memory: add memory_region_to_address()
Date: Thu, 21 Mar 2013 11:09:05 +0000

On 21 March 2013 11:05, Alexander Graf <address@hidden> wrote:
> On 21.03.2013, at 12:01, Peter Maydell wrote:
>> On 21 March 2013 10:59, Alexander Graf <address@hidden> wrote:
>>> On 21.03.2013, at 11:53, Peter Maydell wrote:
>>>> 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".
>>> Same thing here. The question is how the kvm-vgic code in QEMU
>>> finds out where it got mapped to. Scott adds this patch to do
>>> this, but I'd assume you have some other way :)
>> Hmm? The kvm-vgic code in QEMU doesn't need to know where it
>> lives. We have to tell the kernel so it can map its bits of
>> registers in at the right place, that's all.
> The kvm-vgic code in QEMU needs to tell the kernel, no? For
> that, it needs to know what to tell the kernel.

No. As I explained earlier, all the kvm-vgic code needs to do
is call kvm_arm_register_device(). That code in kvm.c then takes
care of telling the kernel. hw/kvm/arm_gic.c itself never knows or
needs to know where it's mapped. This is the whole point of the
mechanism involving notifiers.

> This patch adds a function that allows kvm-openpic to fetch its
> base flat address from the MemoryListener.

This sounds to me like the wrong way to do it -- it's board
models that decide where devices are mapped and the device
code itself shouldn't have to know where it has been mapped.

-- PMM

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