Re: [Qemu-devel] [PATCH 1/2] pc-dimm: No numa option shouldn't break hotplug memory feature

[Tang Chen]

Hi Igor,

On 09/19/2014 08:26 PM, Igor Mammedov wrote:
> On Wed, 17 Sep 2014 16:32:20 +0800
> Hu Tao <address@hidden> wrote:
>
> > On Tue, Sep 16, 2014 at 06:39:15PM +0800, zhanghailiang wrote:
> > > If we do not configure numa option, memory hotplug should work as well.
> > > It should not depend on numa option.
> > >
> > > Steps to reproduce:
> > > (1) Start VM: qemu-kvm -m 1024,slots=4,maxmem=8G
> > > (2) Hotplug memory
> > > It will fail and reports:
> > > "'DIMM property node has value 0' which exceeds the number of numa nodes: 0"
> > I rememberd Tang Chen had a patch for this bug, this is what Andrey suggested:
> >
> >    I thnk that there should be no
> >    cases when dimm is plugged (and check from patch is fired up) without
> >    actually populated NUMA, because not every OS will workaround this by
> >    faking the node.
> This doesn't take in to account that dimm device by itself has nothing to do
> with numa (numa is just optional property of its representation in ACPI land
> and nothing else).
>
> In case initial memory is converted to dimm devices, qemu can be
> started without numa option and it still must work.
>
> So I'm in favor of this path.

I just did some tests. Even if I modify qemu code and make hotpluggable 
bit in SRAT 0,
memory hotplug will still work on Linux guest, which means Linux kernel 
doesn't check
SRAT info after system is up when doing memory hotplug.

I did the following modification in hw/i386/acpi-build.c
-    ram_addr_t hotplugabble_address_space_size =
-        object_property_get_int(OBJECT(pcms), PC_MACHINE_MEMHP_REGION_SIZE,
-                                NULL);
+    ram_addr_t hotplugabble_address_space_size = 0;

And when the guest is up, no memory should be hotpluggable, I think. But 
I hot-added
memory successfully.

IMHO, I think memory hotplug should based on ACPI on Linux. And SRAT 
tells system
which memory ranges are hotpluggable, and we should follow it. So I 
think Linux kernel
has some problem in this issue.

I'd like to fix it like this:

1. Send patches to make Linux kernel to check SRAT info when doing 
memory hotplug.
    (Now, SRAT is only checked at boot time.)

2. In qemu, when users gave a memory hotplug option without NUMA 
options, we create
    node0 and node1, and make node1 hotpluggable.
    This is because when using MOVABLE_NODE, node0 in which the kernel 
resides in should
    not be hotpluggable.
    Of course, make part of memory in node0 hotpluggable is OK, but on 
a real machine, no
    one will do this, I think. So I suggest above idea.

Thanks. :)

> > https://lists.nongnu.org/archive/html/qemu-devel/2014-08/msg04587.html
> >
> > Have you tested this patch with Windows guest?
> >
> > Regards,
> > Hu
>
> .