Re: [Qemu-devel] [Qemu-ppc] [PATCH 2/2] PPC: openpic_kvm: Filter region callbacks based on memory region offset

[address@hidden]

> -----Original Message-----
> From: Wood Scott-B07421
> Sent: Friday, September 05, 2014 6:47 PM
> To: Purcareata Bogdan-B43198
> Cc: address@hidden; Caraman Mihai Claudiu-B02008; address@hidden
> Subject: Re: [Qemu-ppc] [PATCH 2/2] PPC: openpic_kvm: Filter region callbacks
> based on memory region offset
> 
> On Wed, 2014-09-03 at 14:36 -0400, Bogdan Purcareata wrote:
> > This is done due to the fact that the kvm-openpic region_{add,del} callbacks
> > can be invoked for sections generated from other memory regions as well.
> These
> > callbacks should handle only requests for the kvm-openpic memory region.
> >
> > The patch fixes a bug on target-ppc occuring when the "e500-pci-bar0" memory
> > region is added. This memory region registers an alias to the "e500-ccsr"
> memory
> > region, which further contains the "kvm-openpic" subregion. Due to this
> alias,
> > the kvm_openpic_region_add is called once more, with an offset within the
> > "e500-pci-bar" memory region. This generates the remapping of the
> > in-kernel MPIC at a wrong offset.
> 
> OK, so the problem is that we really do have the MPIC mapped in two
> locations (and the kernel API only lets us map one of them).  It would
> be nice if the MemoryRegionSection struct indicated the alias being
> represented rather than needing to recalculate the non-aliased address.

Not sure I fully understand the terminology of the alias being represented. In 
fact, out of the two mentioned in the previous mail, "e500-pci-bar0" and 
"e500-ccsr", I don't know which one is the "alias".

If it's the former, thus "e500-pci-bar0" is an alias for "e500-ccsr", this 
information could be propagated down to the MemoryRegionSection. However, 
according to [1], "aliases may point to any type of region, including other 
aliases". So if the MemoryRegionSection has been built by going through a chain 
of aliases, all this information must be included in the structure.

If it's the latter, thus "e500-ccsr" is an alias for "e500-pci-bar0", we can 
get to it in the current model as well. For our specific case, the 
MemoryRegionSection points to the "kvm-openpic" MemoryRegion, which in turn 
points to "e500-ccsr" as its parent (container).

What do you think?

[1] http://git.qemu.org/?p=qemu.git;a=blob;f=docs/memory.txt

Thanks,
Bogdan P.