Re: [PATCH v2] hw/arm/virt: Expose empty NUMA nodes through ACPI

[Jonathan Cameron]

On Wed, 17 Nov 2021 19:08:28 +0100
David Hildenbrand <david@redhat.com> wrote:

> On 17.11.21 15:30, Jonathan Cameron wrote:
> > On Tue, 16 Nov 2021 12:11:29 +0100
> > David Hildenbrand <david@redhat.com> wrote:
> >   
> >>>>
> >>>> Examples include exposing HBM or PMEM to the VM. Just like on real HW,
> >>>> this memory is exposed via cpu-less, special nodes. In contrast to real
> >>>> HW, the memory is hotplugged later (I don't think HW supports hotplug
> >>>> like that yet, but it might just be a matter of time).    
> >>>
> >>> I suppose some of that maybe covered by GENERIC_AFFINITY entries in SRAT
> >>> some by MEMORY entries. Or nodes created dynamically like with normal
> >>> hotplug memory.
> >>>     
> >   
> 
> Hi Jonathan,
> 
> > The naming of the define is unhelpful.  GENERIC_AFFINITY here corresponds
> > to Generic Initiator Affinity.  So no good for memory. This is meant for
> > representation of accelerators / network cards etc so you can get the NUMA
> > characteristics for them accessing Memory in other nodes.
> > 
> > My understanding of 'traditional' memory hotplug is that typically the
> > PA into which memory is hotplugged is known at boot time whether or not
> > the memory is physically present.  As such, you present that in SRAT and 
> > rely
> > on the EFI memory map / other information sources to know the memory isn't
> > there.  When it is hotplugged later the address is looked up in SRAT to 
> > identify
> > the NUMA node.  
> 
> in virtualized environments we use the SRAT only to indicate the hotpluggable
> region (-> indicate maximum possible PFN to the guest OS), the actual present
> memory+PXM assignment is not done via SRAT. We differ quite a lot here from
> actual hardware I think.
> 
> > 
> > That model is less useful for more flexible entities like virtio-mem or
> > indeed physical hardware such as CXL type 3 memory devices which typically
> > need their own nodes.
> > 
> > For the CXL type 3 option, currently proposal is to use the CXL table 
> > entries
> > representing Physical Address space regions to work out how many NUMA nodes
> > are needed and just create extra ones at boot.
> > https://lore.kernel.org/linux-cxl/163553711933.2509508.2203471175679990.stgit@dwillia2-desk3.amr.corp.intel.com
> > 
> > It's a heuristic as we might need more nodes to represent things well kernel
> > side, but it's better than nothing and less effort that true dynamic node 
> > creation.
> > If you chase through the earlier versions of Alison's patch you will find 
> > some
> > discussion of that.
> > 
> > I wonder if virtio-mem should just grow a CDAT instance via a DOE?
> > 
> > That would make all this stuff discoverable via PCI config space rather 
> > than ACPI
> > CDAT is at:
> > https://uefi.org/sites/default/files/resources/Coherent%20Device%20Attribute%20Table_1.01.pdf
> > but the table access protocol over PCI DOE is currently in the CXL 2.0 spec
> > (nothing stops others using it though AFAIK).
> > 
> > However, then we'd actually need either dynamic node creation in the OS, or
> > some sort of reserved pool of extra nodes.  Long term it may be the most
> > flexible option.  
> 
> 
> I think for virtio-mem it's actually a bit simpler:
> 
> a) The user defined on the QEMU cmdline an empty node
> b) The user assigned a virtio-mem device to a node, either when 
>    coldplugging or hotplugging the device.
> 
> So we don't actually "hotplug" a new node, the (possible) node is already 
> known
> to QEMU right when starting up. It's just a matter of exposing that fact to 
> the
> guest OS -- similar to how we expose the maximum possible PFN to the guest OS.
> It's seems to boild down to an ACPI limitation.
> 
> Conceptually, virtio-mem on an empty node in QEMU is not that different from
> hot/coldplugging a CPU to an empty node or hot/coldplugging a DIMM/NVDIMM to
> an empty node. But I guess it all just doesn't work with QEMU as of now.

A side distraction perhaps, but there is a code first acpi proposal to add
a 'softer' form of CPU hotplug 
https://bugzilla.tianocore.org/show_bug.cgi?id=3706

Whilst the reason for that proposal was for arm64 systems where there is no 
architected
physical hotplug, it might partly solve the empty node question for CPUs.  They 
won't
be empty, there will simply be CPUs that are marked as 'Online capable'.

> 
> In current x86-64 code, we define the "hotpluggable region" in 
> hw/i386/acpi-build.c via
> 
>       build_srat_memory(table_data, machine->device_memory->base,
>                         hotpluggable_address_space_size, nb_numa_nodes - 1,
>                         MEM_AFFINITY_HOTPLUGGABLE | MEM_AFFINITY_ENABLED);
> 
> So we tell the guest OS "this range is hotpluggable" and "it contains to
> this node unless the device says something different". From both values we
> can -- when under QEMU -- conclude the maximum possible PFN and the maximum
> possible node. But the latter is not what Linux does: it simply maps the last
> numa node (indicated in the memory entry) to a PXM
> (-> drivers/acpi/numa/srat.c:acpi_numa_memory_affinity_init()).
yeah.  There is nothing in ACPI that says there can't be holes in the node 
numbering
so Linux does a remapping as you point out.

> 
> 
> I do wonder if we could simply expose the same hotpluggable range via 
> multiple nodes:

Fairly sure the answer to this is no.  You'd have to indicate different ranges 
and
then put the virtio-mem in the right one.  Now I can't actually find anywhere 
in the
ACPI spec that says that but I'm 99% sure Linux won't like and I'm fairly sure 
if we
query it with ACPI folks the answer will be a no you can't don't that.


> 
> diff --git a/hw/i386/acpi-build.c b/hw/i386/acpi-build.c
> index a3ad6abd33..6c0ab442ea 100644
> --- a/hw/i386/acpi-build.c
> +++ b/hw/i386/acpi-build.c
> @@ -2084,6 +2084,22 @@ build_srat(GArray *table_data, BIOSLinker *linker, 
> MachineState *machine)
>       * providing _PXM method if necessary.
>       */
>      if (hotpluggable_address_space_size) {
> +        /*
> +         * For the guest to "know" about possible nodes, we'll indicate the
> +         * same hotpluggable region to all empty nodes.
> +         */
> +        for (i = 0; i < nb_numa_nodes - 1; i++) {
> +            if (machine->numa_state->nodes[i].node_mem > 0) {
> +                continue;
> +            }
> +            build_srat_memory(table_data, machine->device_memory->base,
> +                              hotpluggable_address_space_size, i,
> +                              MEM_AFFINITY_HOTPLUGGABLE | 
> MEM_AFFINITY_ENABLED);
> +        }
> +        /*
> +         * Historically, we always indicated all hotpluggable memory to the
> +         * last node -- if it was empty or not.
> +         */
>          build_srat_memory(table_data, machine->device_memory->base,
>                            hotpluggable_address_space_size, nb_numa_nodes - 1,
>                            MEM_AFFINITY_HOTPLUGGABLE | MEM_AFFINITY_ENABLED);
> 
> 
> Of course, this won't make CPU hotplug to empty nodes happy if we don't have
> mempory hotplug enabled for a VM. I did not check in detail if that is valid
> according to ACPI -- Linux might eat it (did not try yet, though).
> 
>