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

[zhanghailiang <zhang.zhanghailiang@huawei.com>]

On 2014/9/22 17:03, Tang Chen wrote:
> Hi Igor,
>
> On 09/19/2014 08:26 PM, Igor Mammedov wrote:
>> On Wed, 17 Sep 2014 16:32:20 +0800
>> Hu Tao <hutao@cn.fujitsu.com> 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

It seems that if we support pc-dimm memory without numa option in startup,
we still need this patch...

>> 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.

In this case, guest os will see two numa node even if we don't configure any numa option.

>      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.

In actual usage scenario, usually we use the guest numa cooperating with the host numa,
It may be strange that we can not hotplug memory to guest node0...

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