Re: [Qemu-devel] Regression (?) due to c4177479 ('spapr: make sure RMA is in first mode of first memory node')

[Benjamin Herrenschmidt]

On Fri, 2014-04-18 at 08:43 +1000, Alexey Kardashevskiy wrote:
> On 04/18/2014 06:25 AM, Nishanth Aravamudan wrote:
> > Hi Alexey,
> > 
> > Prior to the $SUBJECT commit, we could present memoryless node0s to
> > guests. Now, we indicate that we don't have the requisite 128M for the
> > RMA if node 0 has no memory. Note that a memoryless node0 is possible
> > under PowerVM (but not predictably present) so I was hoping to use KVM
> > to test relevant fixes for memoryless nodes.
> > 
> > I think this change is a misinterpretation of the PAPR standard, though.
> > Yes, the RMA must be in the first block of memory, but that isn't
> > necessarily on node 0. The topology of a PAPR-compliant guest does not
> > require a node 0 (and in fact, under PowerVM, Linux doesn't actually
> > require node 0 either, but it would under KVM).
> > 
> > Thoughts? I suppose it's fine to say that node 0 must be sufficiently
> > populated under KVM -- there's not really a reason to not have memory on
> > a given node (except maybe ballooning). I can keep the commit reverted
> > locally for testing purposes. Just wanted to see if the semantic change
> > was intentional.
> 
> 
> PAPR spec 2.7:
> C.6.6 Memory Node
> ===
> This section defines the PAPR modifications to the OF /memory node. In
> PAPR, the memory allocated to an OS image
> may be divided into a number of allocation units called “regions” or
> “Logical Memory Blocks (LMB). An OS image
> may be dynamically allocated additional regions or may be asked to release
> regions. Each LMB is either represented in
> the device tree by its own /memory node or by an entry in
> /ibm,dynamic-reconfiguration-memory nodes
> (see Section C.6.6.2‚ “ibm,dynamic-reconfiguration-memory‚” on page 1089).
> The /memory node that refers to the
> storage starting at real address zero (“reg” property starting at the value
> zero) always remains allocated to an OS image.
> 
> The client program is initially loaded into this storage, called the RMA,
> that is represented by the first value of the
> “reg” property of this first /memory node.
> ===
> 
> The last sentence is why the change was made. It does not say "first
> populated" node. I am adding Ben as he had very strong opinion about this
> thing.

You are confusing device-tree node with NUMA nodes.

Yes, it must be the LMB at address 0, which is the /memory node, but
that doesn't have to be NUMA node 0.

Cheers,
Ben.