Re: [PATCH] hw/arm/boot: Use NUMA node ID in memory node name

[Gavin Shan]

Hi Drew,

On 6/1/21 5:50 PM, Andrew Jones wrote:
> On Tue, Jun 01, 2021 at 03:30:04PM +0800, Gavin Shan wrote:
> > We possibly populate empty nodes where memory isn't included and might
> > be hot added at late time. The FDT memory nodes can't be created due
> > to conflicts on their names if multiple empty nodes are specified.
> > For example, the VM fails to start with the following error messages.
> >
> >    /home/gavin/sandbox/qemu.main/build/qemu-system-aarch64          \
> >    -accel kvm -machine virt,gic-version=host                        \
> >    -cpu host -smp 4,sockets=2,cores=2,threads=1 -m 1024M,maxmem=64G \
> >    -object memory-backend-ram,id=mem0,size=512M                     \
> >    -object memory-backend-ram,id=mem1,size=512M                     \
> >    -numa node,nodeid=0,cpus=0-1,memdev=mem0                         \
> >    -numa node,nodeid=1,cpus=2-3,memdev=mem1                         \
> >    -numa node,nodeid=2                                              \
> >    -numa node,nodeid=3                                              \
> >      :
> >    -device virtio-balloon-pci,id=balloon0,free-page-reporting=yes
> >
> >    qemu-system-aarch64: FDT: Failed to create subnode /memory@80000000: \
> >                         FDT_ERR_EXISTS
> >
> > This fixes the issue by using NUMA node ID or zero in the memory node
> > name to avoid the conflicting memory node names. With this applied, the
> > VM can boot successfully with above command lines.
> >
> > Signed-off-by: Gavin Shan <gshan@redhat.com>
> > ---
> >   hw/arm/boot.c | 7 ++++++-
> >   1 file changed, 6 insertions(+), 1 deletion(-)
> >
> > diff --git a/hw/arm/boot.c b/hw/arm/boot.c
> > index d7b059225e..3169bdf595 100644
> > --- a/hw/arm/boot.c
> > +++ b/hw/arm/boot.c
> > @@ -432,7 +432,12 @@ static int fdt_add_memory_node(void *fdt, uint32_t acells, 
> > hwaddr mem_base,
> >       char *nodename;
> >       int ret;
> >   
> > -    nodename = g_strdup_printf("/memory@%" PRIx64, mem_base);
> > +    if (numa_node_id >= 0) {
> > +        nodename = g_strdup_printf("/memory@%d", numa_node_id);
> > +    } else {
> > +        nodename = g_strdup("/memory@0");
> > +    }
> > +
> >       qemu_fdt_add_subnode(fdt, nodename);
> >       qemu_fdt_setprop_string(fdt, nodename, "device_type", "memory");
> >       ret = qemu_fdt_setprop_sized_cells(fdt, nodename, "reg", acells, mem_base,

[...]

> Is it conventional to use the unit-address like this? If so, can you point
> out where that's documented? If it's not conventional, then we shouldn't
> do it. And then I'm not sure what we should do in this case. Here's a
> couple links I found, but they don't really help...
>
> https://devicetree-specification.readthedocs.io/en/latest/chapter2-devicetree-basics.html#sect-node-names
> https://devicetree-specification.readthedocs.io/en/latest/chapter3-devicenodes.html#memory-node

As stated in the document (section 2.2.1.1). It's conventional to take the first
address of 'reg' property as unit-address, but it's not mandatory as I 
understand:

(1) In section 2.2.1.1, the bus can specify additional format to unit-address.
(2) The device node name isn't used to identify the device node in Linux kernel.
    They are actually identified by 'device_type' property.
    (drivers/of/fdt.c::early_init_dt_scan_memory())

I think it's still nice to include the unit-address in meory node's name. For 
the
conflicting nodes, we add more suffix like below. I can update the code in v2 if
it's preferred way to go.

   memory@0
   memory@0-0                               # For empty NUMA node
   memory@0-1                               # For empty NUMA node
   memory@80000000
   memory@80000000-0                        # For empty NUMA node
   memory@80000000-1                        # For empty NUMA node

---

https://devicetree-specification.readthedocs.io/en/latest/chapter2-devicetree-basics.html#sect-node-names

The unit-address must match the first address specified in the reg property of 
the node.
If the node has no reg property, the @unit-address must be omitted and the 
node-name
alone differentiates the node from other nodes at the same level in the tree. 
The binding
for a particular bus may specify additional, more specific requirements for the 
format
of reg and the unit-address.

Thanks,
Gavin