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

[Gavin Shan]

Hi Drew,

On 6/2/21 9:36 PM, Andrew Jones wrote:
> On Wed, Jun 02, 2021 at 11:09:32AM +1000, Gavin Shan wrote:
> > 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.
>
> The bus type we're using is the physical memory map. Does it allow this
> use of the unit-address? I still haven't seen that documented anywhere.
>
> > (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())
>
> This doesn't matter. We can't change DT node name formats just because
> they may not impact Linux. We need to follow the DT standard and the Linux
> convention.
>
> > 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.
>
> I don't think we should change the semantics of the unit address at all,
> not without either a) finding a document that says our use is OK or b)
> getting it documented in the Linux kernel first.

[...]

I've sent one separate mail to check with Rob Herring. Hopefully he have
ideas as he is maintaining linux FDT subsystem. You have been included to
that thread. I didn't find something meaningful to this thread after doing
some google search either.

Yes, I agree with you we need to follow the specification strictly. It seems
it's uncertain about the 'physical memory map' bus binding requirements.

Thanks,
Gavin