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

From: Igor Mammedov
Subject: Re: [PATCH v2] hw/arm/virt: Expose empty NUMA nodes through ACPI
Date: Fri, 12 Nov 2021 14:27:51 +0100

On Wed, 10 Nov 2021 12:01:11 +0100
David Hildenbrand <david@redhat.com> wrote:

> On 10.11.21 11:33, Igor Mammedov wrote:
> > On Fri, 5 Nov 2021 23:47:37 +1100
> > Gavin Shan <gshan@redhat.com> wrote:
> >   
> >> Hi Drew and Igor,
> >>
> >> On 11/2/21 6:39 PM, Andrew Jones wrote:  
> >>> On Tue, Nov 02, 2021 at 10:44:08AM +1100, Gavin Shan wrote:    
> >>>>
> >>>> Yeah, I agree. I don't have strong sense to expose these empty nodes
> >>>> for now. Please ignore the patch.
> >>>>    
> >>>
> >>> So were describing empty numa nodes on the command line ever a reasonable
> >>> thing to do? What happens on x86 machine types when describing empty numa
> >>> nodes? I'm starting to think that the solution all along was just to
> >>> error out when a numa node has memory size = 0...  
> > 
> > memory less nodes are fine as long as there is another type of device
> > that describes  a node (apic/gic/...).
> > But there is no way in spec to describe completely empty nodes,
> > and I dislike adding out of spec entries just to fake an empty node.
> >   
> There are reasonable *upcoming* use cases for initially completely empty
> NUMA nodes with virtio-mem: being able to expose a dynamic amount of
> performance-differentiated memory to a VM. I don't know of any existing
> use cases that would require that as of now.
> 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.

> The same should be true when using DIMMs instead of virtio-mem in this
> example.
> >   
> >> Sorry for the delay as I spent a few days looking into linux virtio-mem
> >> driver. I'm afraid we still need this patch for ARM64. I don't think x86  
> > 
> > does it behave the same way is using pc-dimm hotplug instead of virtio-mem?
> > 
> > CCing David
> > as it might be virtio-mem issue.  
> Can someone share the details why it's a problem on arm64 but not on
> x86-64? I assume this really only applies when having a dedicated, empty
> node -- correct?
> > 
> > PS:
> > maybe for virtio-mem-pci, we need to add GENERIC_AFFINITY entry into SRAT
> > and describe it as PCI device (we don't do that yet if I'm no mistaken).  
> virtio-mem exposes the PXM itself, and avoids exposing it memory via any
> kind of platform specific firmware maps. The PXM gets translated in the
> guest accordingly. For now there was no need to expose this in SRAT --
> the SRAT is really only used to expose the maximum possible PFN to the
> VM, just like it would have to be used to expose "this is a possible node".
> Of course, we could use any other paravirtualized interface to expose
> both information. For example, on s390x, I'll have to introduce a new
> hypercall to query the "device memory region" to detect the maximum
> possible PFN, because existing interfaces don't allow for that. For now
> we're ruinning SRAT to expose "maximum possible PFN" simply because it's
> easy to re-use.
> But I assume that hotplugging a DIMM to an empty node will have similar
> issues on arm64.
> >   
> >> has this issue even though I didn't experiment on X86. For example, I
> >> have the following command lines. The hot added memory is put into node#0
> >> instead of node#2, which is wrong.  
> I assume Linux will always fallback to node 0 if node X is not possible
> when translating the PXM.

I tested how x86 behaves, with pc-dimm, and it seems that
fc43 guest works only sometimes.
  -numa node,memdev=mem,cpus=0 -numa node,cpus=1 -numa node -numa node

1: hotplug into the empty last node creates a new node dynamically 
2: hotplug into intermediate empty node (last-1) is broken, memory goes into 
the first node

We should check if it possible to fix guest instead of adding bogus SRAT 

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