[Qemu-devel] issue about numa configure

[Qemu-devel] issue about numa configure

[linzhecheng]

Hi, all
I found that qemu has a constraint in function numa_node_parse now:
     If (node->has_memdev != have_memdevs) {
         Error_setg(errp, "qemu: memdev option must be specified for either "
                    "all or no nodes");
         Return;
     }
This restricts us from being able to configure an empty numa node (without memory and cpus). But if I delete these codes, I can start a VM with cmdline:
qemu-system-x86_64 --enable-kvm  -m size=2G,slots=256,maxmem=300G -smp 2,maxcpus=4,sockets=4,cores=1,threads=1 -numa node,nodeid=0,cpus=0-1,mem=2048 -numa node,nodeid=1,cpus=2-3 ...
We can see only one numa node inside the VM(I have tested both linux and windows) after beginning.
And if I hot-plug the dimm memory devices into the empty node, vm will present a new numa node inside and the new memory is online then.
I'm wondering if you have any related issue before? Or can we remove this constraint?
Looking forward to your answers, thanks.

Re: [Qemu-devel] issue about numa configure

[Eduardo Habkost]

Hi,

On Thu, Jul 19, 2018 at 09:21:36AM +0000, linzhecheng wrote:
> Hi, all
> I found that qemu has a constraint in function numa_node_parse now:
>      If (node->has_memdev != have_memdevs) {
>          Error_setg(errp, "qemu: memdev option must be specified for either "
>                     "all or no nodes");
>          Return;
>      }
> This restricts us from being able to configure an empty numa node (without memory and cpus). But if I delete these codes, I can start a VM with cmdline:
> qemu-system-x86_64 --enable-kvm  -m size=2G,slots=256,maxmem=300G -smp 2,maxcpus=4,sockets=4,cores=1,threads=1 -numa node,nodeid=0,cpus=0-1,mem=2048 -numa node,nodeid=1,cpus=2-3 ...
> We can see only one numa node inside the VM(I have tested both linux and windows) after beginning.
> And if I hot-plug the dimm memory devices into the empty node, vm will present a new numa node inside and the new memory is online then.
> I'm wondering if you have any related issue before? Or can we remove this constraint?
> Looking forward to your answers, thanks.

The check is there because memory_region_allocate_system_memory()
doesn't know how to allocate memory correctly if only some nodes
use memdev.

I wouldn't remove the check completely, but just skip it if
"mem=0" is specified explicitly.

-- 
Eduardo