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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: Sat, 23 Mar 2013 11:24:49 +0000

On 22 March 2013 22:05, Scott Wood <address@hidden> wrote:
> On 03/22/2013 08:08:57 AM, Peter Maydell wrote:
>> The memory API flattens the tree of memory regions down into a flat
>> view of the address space. These callbacks get called for the
>> final flattened view (so you'll never see a pure container in the
>> callback, only leaves). The callbacks happen for every region which
>> appears in the address space, in linear order. When an update happens
>> memory.c identifies the changes between the old flat view and the
>> new one and calls callbacks appropriately.
> OK, so .add and .del will be sufficient to capture any manipulation that
> would affect whether and where the region we care about is mapped?

Yes. Note that if the board (brokenly) maps the region so it is
'hidden' by another region, this manifests as a .del [since it
is no longer accessible]. Also I think if the board maps something
small on top and in the middle of the region you get an add for
each of the partially visible fragments. Personally I'm happy to
not worry about either of these cases on the basis that they would
be board model bugs.

>> This code isn't the
>> first use of the memory API listeners, so it's all well-tested code.
> Sure, I'm not suggesting the code doesn't work -- just trying to understand
> how, so I know I'm using it properly.  The implementation is a bit opaque
> (to me at least), and the listener callbacks aren't documented the way the
> normal API functions are.

Yeah, it would I guess be good to add doc comments for all the fields
in struct MemoryListener describing the semantics of the callbacks.

>> > The MPIC's memory listener isn't that much code... I'm not sure
>> > there's a great need for a central KVM registry.
>> Well, nor is the ARM memory listener, but why have two bits of
>> code doing the same thing when you could have one?
> They're not doing quite the same thing, though, and the effort required to
> unify them is non-zero.  The two main issues are the way that the address is
> communicated to KVM, and the ability to change the mapping after the guest
> starts.

Ah, guest-programmable mappings are a real use case and not a hypothetical?
Do we run into synchronisation issues with making sure that QEMU and
the kernel both agree simultaneously about where the mapping is?
Can the mapping be different between different CPU cores? [let's
hope not :-)] Is the mapping controlled by a register within the
mapping itself, or is there some separate non-moving register which
defines the location of the mappable registers?

-- PMM

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