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

[Andrew Jones <drjones@redhat.com>]

On Tue, Jun 22, 2021 at 06:53:41PM +1000, Gavin Shan wrote:
> Hi Drew,
> 
> On 6/3/21 2:48 PM, Gavin Shan wrote:
> > 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,
> 
> [...]
> 
> > 
> > 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.
> > 
> 
> I didn't get expected answers from device-tree experts.

What response did you get? Can you please provide a link to the discussion?

> After rethinking about it,
> I plan to fix this like this way, but please let me know if it sounds sensible
> to you.
> 
> The idea is to assign a (not overlapped) dummy base address to each memory
> node in the device-tree. The dummy is (last_valid_memory_address + NUMA ID).
> The 'length' of the 'reg' property in the device-tree nodes, corresponding
> to empty NUMA nodes, is still zero. This ensures the nodes are still invalid
> until memory is added to these nodes.
> 
> I had the temporary patch for the implementation. It works fine and VM can
> boot up successfully.

I would like to be sure that the kernel developers for NUMA, memory
hotplug, and devicetree specifications are all happy with the approach
before adding it to QEMU.

Thanks,
drew